So I think I may have a vacuum leak. I have the whining sound while driving, however I wasn't keen to do the carb cleaner test that most people recommend since it can prove to be dangerous.

I did a compression test on my car a few weeks ago and I got 150/151 across all 4. My friend who owns a 1990 JDM-imported MR2 also got 150 across all 4. My only issue with that is since mine is a Gen 3 3sge, and my compression ratio should be higher, I'm pretty sure I should be getting more than 150/151 across the board which could also be another indication of a vacuum leak.

I had a quick look with a mate and it sounds like it could coming from under/around the intake manifold. Since I've never dealt with a vacuum leak, what should I be looking out for? And are there any common issues with the MR2 in this area that I should look at first?

Was comp. test done with engine at operating temp. and all plugs out and throttle wide open?  150 is on the low end of the scale. Would be better if it was 175.  But even is good.
There are a number of vacuum hoses and vacuum switching valves mounted low down on inlet manifold along with the vacuum reservoir.  Check all of these for splits and proper fit and CORRECT routing.  A vacuum leak is usually most audible at idle.   You don't know whats been buggered about with so be suspicious of every thing. (Gaskets etc.)
Im not familiar with Bosch plugs but the NGK BKR6EYA  set to 0.8mm is the recommended plug.
98 Octane is the fuel to use but there is certainly something really out of whack with your engine if it is using as much as you say for only 350 kms.   Do an accurate fuel consumption check  Litres. Kms.
 Check condition of the dissy cap and rotor . Check resistance of high tension leads including the master . Don't just buy new rubbish ones. Tota  original are better.
 Check ignition timing and cam timing as per BGB
If you get these few things corrected back to original you should be pleasantly surprised at how well it runs and it should give you around 600 kms for 40 - 45 litres of 98 octane.
Cheers.

Falcon
Was comp. test done with engine at operating temp. and all plugs out and throttle wide open?  150 is on the low end of the scale. Would be better if it was 175.  But even is good.
There are a number of vacuum hoses and vacuum switching valves mounted low down on inlet manifold along with the vacuum reservoir.  Check all of these for splits and proper fit and CORRECT routing.  A vacuum leak is usually most audible at idle.   You don't know whats been buggered about with so be suspicious of every thing. (Gaskets etc.)
Im not familiar with Bosch plugs but the NGK BKR6EYA  set to 0.8mm is the recommended plug.
98 Octane is the fuel to use but there is certainly something really out of whack with your engine if it is using as much as you say for only 350 kms.   Do an accurate fuel consumption check  Litres. Kms.
 Check condition of the dissy cap and rotor . Check resistance of high tension leads including the master . Don't just buy new rubbish ones. Tota  original are better.
 Check ignition timing and cam timing as per BGB
If you get these few things corrected back to original you should be pleasantly surprised at how well it runs and it should give you around 600 kms for 40 - 45 litres of 98 octane.
Cheers.

Engine comp test was done after maybe a 10 min drive so was a little warm, but I kept the other 3 plugs in every time. Throttle was closed also. Guess I should re-test that then, didn't really know what I was doing.

As for the vacuum leak, it's very noticeable which is what lead me on to it. Since there's so many possibilities, would I be better off throwing it at a mechanic for the time being as my knowledge is lacking?

How would I go about during an accurate fuel consumption check? I already know it's pretty poor, as it's doing 350km to a tank.

Never really looked at the dissy cap, rotor or leads so I'll give those a look. Those 3 are completely new to me (learning as I go) so I'll have to Google search those. Ignition timing and cam timing as per BGB?

So I re-did the compression test and I got 180 across the board. Still not entirely sure on what to check for the vacuum leak.

Also checked the ignition timing and it was spot on according to http://www.imoc.co.uk/tec..al/howto/mk2ttiming.htm

So that leaves high tension leads, cam timing or vacuum leak

Is your engine idling at a higher RPM than normal? 
 
After reading your first post I thought you might be unaware of how a vacuum leak usually presents it’s self.  The most common symptom is the idle speed will increase above it’s normal idle RPM level, how high above normal will depend on how much uncontrolled  air is allowed to enter the engine.  What you might be experiencing is an exhaust leak.  This would explain why your compression test was normal.

kojab
Is your engine idling at a higher RPM than normal? 

After reading your first post I thought you might be unaware of how a vacuum leak usually presents it’s self.  The most common symptom is the idle speed will increase above it’s normal idle RPM level, how high above normal will depend on how much uncontrolled  air is allowed to enter the engine.  What you might be experiencing is an exhaust leak.  This would explain why your compression test was normal.

It does idle a little high I think, I'll double check it tomorrow morning. Also thanks for helping clear it up, everyone starts somewhere. I thought that a certain whine I could hear from the car was perhaps the vacuum leak (air being sucked through small slit = whine, I guess) but apparently that's a gearbox thing. 

An exhaust leak does make a bit of sense from what you've said though. Only thing I wanted to ask, would an exhaust leak lead to sluggish performance and excessive fuel consumption? Because right now, those are the two things I've noticed most when compared to my friend's MR2.

By exhaust leak, do you mean a hole in the exhaust system somewhere or just a gasket gone bad? (again, still learning haha)

So started her up this morning and she idles at 1000-1050rpm, however I did notice the rpm gauge fluctuates a little. Only by 50 or so rpm (so ranging from 950-1050) however. 

Tested the exhaust leak theory by letting it idle and putting two rags over each tail pipe. This method is supposed to block the airway and force the air to come out through the crack, if there is one. Didn't hear anything unusual, it just kinda muffled.  I'll try to get under the car and visually inspect the exhaust system next time the garage is free (should be tomorrow).

Did it only just start doing this or has it been doing it since you bought the car?
When warm it shouldnt be idling at 1000rpm. More like 750/800.

Always I'm pretty sure. I just never noticed it was off (first car was an auto CE lancer so everything felt like an upgrade). It was only recently going for a drive in my mates Mr2 which is supposed to be 10hp weaker I noticed something was off

So just a follow up update, had my mechanic have a look at it. He said he gave it a small service (not sure what it consisted of other than I know he did the fuel filter) and it's running pretty swell now. Could just be placebo though, but it does feel a lot smoother/less hesitant to move. He said he couldn't find any mechanical faults, so he refused to charge me (even for the servicing). So pretty happy with the result.

I'll throw a full tank of 98 in the next few days and see how far that gets to me, hoping to get minimum 550km but we'll see.