Hi mate, your classic Prodrive should put out around 240bhp as standard, though with age you might have lost a few horses from the stable. Unfortunately there is no way that anyone on here can tell you if your car is under achieving.
A few ways would be to rolling road your scoob to see how much power its producing, another is give it a good service and see if that helps. You could try going out in a fellow owners version, but this is a long shot as i very much doubt two scoobs would be alike, what with mods being done to most owners cars.
Or you could fit gauges and make sure turbo pressure is correct etc etc.
Or it could be your conception of what you thought a Subaru should go like? What did you have before the scoob?