so you're running a classic shape rex?

i'm not sure what the fuel maps are for the classic shape and MY01 onwards WRX are, but that may contribute to the fuel used, on top of the heavy foot.

650km from a tank country driving is achievable, as there's less stop/starts and just one constant speed.

some of the guys here on this thread has either 2.5L rex, upgraded injectors/fuel pumps, and pushing a bit more kw as well. plus, its better to run slightly richer than lean