I run an FSP Impreza wagon. I can fill in some blanks anyway. I've got my list of modifications at http://www.jenandneil.com/28fsp.
The car weighs 2680 lbs and that's with heavy-ish STi seats and steering wheel, 22lb alloy wheels, power windows/locks, and the passenger airbag-equipped dashboard. I imagine swapping in a lighter set of seats and steering wheel without an airbag, the older dashboards without an airbag, lightweight wheels, and manual windows/locks could yield another almost 50 lbs. A lightweight exhaust could help some more. The weight split is 55/45 F/R.
The engine is built, but there's no engine management. So I'm getting about 112 HP and 120 lb-ft of torque measured with a G-Tech Pro/SS. The torque peaks at about 4000-4200 RPMs and starts to really trail off around 5300 until redline at 6200. I'm running 245/40R17 tires this year, but the 275/35R15 are definitely a contender. I'm sure you can be pretty competitive on both sets.
I haven't run with my new tires yet. On my old 205/45R16 tires, I was getting lateral Gs between 1.2 and 1.3. I could always turn faster to the left but hopefully my new alignment will repair that. I can accelerate about 0.3G and decelerate at almost 1G.
Hope that helps. I'm only 0.3L displacement down from a Impreza 2.5RS and otherwise, there's nothing to distinguish my car from a DSP Impreza, except the weight balance is a little in my wagon's favor.
-N
#128 FSP - '99 Soloru Wagon - NER