Fuel efficiency on the wrx is relative
If you want efficiency and poke, a VW Golf GT 1.4 is the way to go

650km to a tank
With a tune, exhaust, can get 120kw atw...2WD

From having an 00' Bugeye, and assuming that your calculations are accurate, it does sound like u are using a bit more than u should(compression test?) A bit of gearbox whine is not uncommon in first and second, do u have any difficulties in quick gear changes? If not I wouldn't worry about paying someone to remove and crack open the box.

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"ALLPAWS" 2014 WRX RS 40 - modding very slowly.
2002 Mitsubishi Airtrek Turbo R - Ralliart Edition (daily)
2010 Nissan GT-R - when the Missus let's me drive it :)
Crotch Rockets . . .

Does a bug eye fuel tank even hold 55 litres?

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Bye bye betty blue. Hello F6.

Yeah they do, I don't even measure my fuel economy otherwise it would upset me. I just enjoy the car

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Bitches love cardigans

Isn't that the TSI model that VW had to replace loads of engines over here due to detonation?

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Bye bye betty blue. Hello F6.

That the v6 that had issues
Mk5 was fine, they tried to over do it from factory with mk6

Back on topic
You can run rich if you're O2 sensors are off, AFM dirty
Chuck a scan gauge on he car and see if any errors come up

Mavx4; Well, my 'turbo charged sports' Saab 9-3 Aero was as quick as my WRX, and used around 8 litres per 100km no matter how I drove it. So I was expecting around the 10 litres per 100km mark on a run. Which from some of the posts I've seen seems to be a fair expectation.

Cheers for that. Will the scan show up errors even in the absence of a CEL?

You don't always get a CEL from my experience

Just a bit of an update.

I filled up my car with BP Ultimate and zeroed the trip recorder, and did a mix of motorway, commuting, and rush hour crawling for a few days. It managed 475kms before the fuel light came on, which works out to a shade over 11L/100km, which is quite a bit better than I'd originally thought and about average for what I've been told elsewhere.

The gearbox whine I mentioned in second is pretty normal according to a Scooby tech I spoke with. Apparently it's when the gears baulk in between changes that you need to worry.

Cheers to everyone who took the time to post!