One thing that you have not looked at is the oxygen sensor which plays a very important part in good fuel economy. This is the only sensor that gives feed back to the ECU so it can decide if the engine is running rich or lean.  Factory ECUs use an integrator which is a bi-product of the oxygen sensor's output (feedback) and it instantly pulls the desired AFR (air fuel ratio) to 14.7:1 which is good for fuel economy.  If the oxygen sensors output is lower than it should be due to old age the engine will be given the wrong feed back and over fueling will occur.  Sometimes an oxygen sensor can be poisoned by contaminants and the oxygen sensors output will by higher than normal causing a lean condition which can also cause all kinds of problems.  So an oxygen sensor's output voltage might be incorrectly high or low but as long as it is moving it is unlikely to set a fault code light.   It is for this reason a faulty oxygen sensor is often overlooked as very few mechanics if any, have tools good enough to test this sensor.
Usually only replaced when a faulty oxygen code is generated due to no output/movement from the sensor and the engine light comes on.
 
What you could do as a test.
 
Disconnect your oxygen sensor at it's 4 pin plug. Reset the ECU buy removing all power by disconnecting your battery for an hour or so.  This should allow the ECU to drop its' learnt memory data.  Reconnect battery and test drive for a full tank of fuel with oxygen sensor disconnected.   Yes the engine light should come on as your oxygen sensor is not seen buy the ECU.
 
Would be interesting to know your result.

mr2y
FYI I was the previous owner. Now that I think about it, I was in the process of getting the thermostat changed when I did the coolant change. Have you had the thermostat replaced? I remember the temperature gauge took way too long to get warm, like 10 minutes. I'm not really clued in on MR2s, I've only ever worked on old school carburetor vehicles but maybe a low temp will cause excessive fuel consumption.

I haven't replaced it, but you're definitely right - the temp gauge takes a million years to reach proper temperature. I'm not sure if it'd affect fuel consumption however - I thought it was more of sensor regarding when engine bay fans should be turned on / a warning "gauge" in case the car overheats. It could, I don't know what I'm talking about, someone might be able to shed some light on it..

I'm definitely thinking its the o2 however - the car smells like crap from the exhaust after I park it after a 5-10min+ drive - not of fuel though. I don't know what 'lean' smells like but I'm feeling its that

I've got a replacement Denso o2 sensor (cheap eBay one), hoping it fits without issue. Need a hand getting it in though..