If you're using that much fuel, sounds like there is a whole lot more wrong with your car than just the supercharger switching in.
 
When did it last get a proper service?  Plugs, leads, air filter etc.  Also, what fuel are you using?  The extra cost of premium grade is more than offset by the extra distance you'll get on each tank full.  PM me if you'd like to chat further, I'm sure you'd want to enjoy your little gem properly . . .

Check your coolant temp sensor! If you're chewing that much fuel something is definitely wrong. Coolant temp sensor acting faulty means the engine still thinks its cold and runs pig rich.
Definitly give it a service, also see if you can get your hands on a bottle of Subaru fuel additive, add it to a full tank of fuel and run it through until your fuel light comes on. it'll clean out your injectors and intake very well.
 
With a factory GZE you shouldn't be getting any worse than around 320km per tank, usually around 380-400km with a blend of spirited driving and occasionally cruising.

Thank you all for the input.

I only use BP ultimate, I would use 100ron if it was easily available here.

It had a major service 2 years ago by the former (mechanic) owner. Yes it is overfuelling with the charger on, and also has a flat spot, she could do with a tune up. The only local shop I could find with the equipment to run a diagnostic on it is an expensive race builder I can't afford right now. She is due for her 200k service, I intend to replace:

Timing belt, water pump, a/c-alt and s/c belts, plugs, leads, pod filter, all fluids. It also needs a new cat and muffler. I was considering doing the head-gasket (+bolts), cam seals etc, but will probably just leave it for now.

It sounds like the temp sensor could be an issue, when it's hot, it often drops the idle speed too low and wants to stall. It is not overheating, did 700km on very hot day without issue. Other than this, and the slight flat spot around 3.5k, it runs beautifully.

It gets around 250-550km per tank depending on conditions, the charger and driving style.

Feral- please do not mistake humility for inexperience. I have owned many cars, including two N/A Aw11's, I have driven professionally for some years. I can and do drive the 4agze off boost, however you seem to be missing the point, which is that I actually LIKE the alternative, mild behaviour of this car for commuting and extended highway cruising, economy aside, it is smoother and sounds nicer. Each to their own.

wire it via a relay, main fused source from the ignition so it can not be left on.  the s/c clutch is negative triggering from the ecu (has a constant 12v+ with ign on).
 
wire it up simular to a spot light relay.  
 
enjoy.

Head to opc.mr2oc.com and download the BGB, usually the '89 one is pretty good.
 
Read up how to put the car in diagnostic mode (involves bridging two connectors on the diag plug in your engine bay), then you will get blinking lights for any sensor fault codes, that's a good place to start.
 
It's pretty easy to explain it here, but I recommend you read the BGB because you will find it's actually pretty easy to give these cars a bit of mechanical love that only an owner can. You can always find a good mechanic but you are paying them for their time to look at the car, whereas you can do it anytime you want and give it good attention to detail. You don't need any fancy diagnostic gear because these cars are pre OBDII etc. so the diag mode is as good as any piece of hardware! The ECU certainly doesn't have any learning when it comes to sensors, no limp home mode etc.
 
If you don't have any sensor issues, then continuing reading to learn how to check things like your TPS, vacuum system etc. so everything is in spec. You'll find a lot to gain about fine tuning the car so it runs as healthily as it can without actually having to replace anything (if diag mode comes up clear that is).
 
For extended highway cruising, the SC is usually switched off anyway, so you aren't silly for liking it. All we are saying is under normal conditions, you shouldn't need to worry about a switch to turn it on during cruising; it'll only ever engage when you put your foot down for overtaking etc.
 
I love sitting my GZE Auto on overdrive so it sits at 2,200rpm doing 110kph down the highway, no SC engaged, such a nice and quiet cruiser (except for wind noise of course).

Feral- thanks, that is precisely and concisely the answer I was looking for! I used to be into 4WD's, so wiring a spotlight is a good analogy.
 
Maj- double thanks, we are getting off-topic but your advice is extremely helpful and much appreciated. I share the same philosophy, unless you can afford to pay top dollar for a high end shop or dedicated (one make/model) specialist, pedantic owner wins every time! I only outsource when I lack the equipment or suitable workspace.
 
Mine has older extractors, sports muffler and hot-dog, not exactly quiet, but not invasive either. After driving on the highway with the charger off she pops and burbles so wonderfully compression braking down to low revs, it gives me an eargasm! Or it did until the baffling came loose. :(
 
I just sold an N/A AW11 that was meant to be my daily, as I can't currently afford to run two cars, so one car with two modes is basically a compromise. Realistically when I can afford it, my second car will be a diesel 4WD for practicality, off-roading and towing the A-dub to track days.
 
I did not realise they were so easy to diagnose, that will help me a lot!

Look into the possibility of the O2 sensor being bad aswell. Just my 2c

4wd's and aw11's.. best of both worlds. 
 

 
 
yeah I did 8 years of high end audio/alarm install which entailed a lot of auto electrical stuff.  if only I could remember it all now...  used to be able to do prity wild shiz with relays.

feral4mr2
4wd's and aw11's.. best of both worlds. 

Indeed.

Here's my AW11 and my old 4WD ;)