The WRX would be a better car to mod if you wanted higher HP, but most people who mod do just basic things. People into serious performance would go for the STi or Evo.
You'd be surprised. There are plenty of modified WRXs making STi or better power. Why? The cars use the same block. If you're going to start modifying the engine, the WRX gives you the same starting point for less money: it doesn't matter that the turbo is smaller, since you're replacing it anyway. 
The WRX is also cheaper to insure than the STi. 
And you can modify it at your leisure, instead of putting down $50 000 or being locked into $50k worth of payments.
I'd love an STi, but from my POV, a modified WRX makes far more sense. Biggest difficulty might be finding an AWD dyno.
Agreed, to achieve STI like performance from a WRX you will need the following:
COBB Stage 2: $1750
AccessPort AWD ProTune: $600
OZ Racing Superleggeras 17 X 7.5 : $1200
Goodyear Eagle F1 Gs-D3 225 / 45 R17 91y : $1100
$4650 CDN is real pricing based on real experiences and not sitting on a couch or chair reading about it. Feeling like your going fast and actually going fast are two different things IMO.
