allow me to answer some of these points. my roommate has an 09 STI gr. i road in it every other day for the last year + and have driven it a few times. that, and the z and subie guys do a lot of stuff together here in kc as well.
the sti has better mod potential to a point... sure, u can bolt on 320whp with the stock turbo. or go rotated and make as much power as u want. HOWEVER, even with the z being na, it can handle high hp MUCH better than sti's. sti's have, and have had forever, ringland issues when u go over 400whp. pretty much garunteed to crack them in a couple weeks at that hp level. then its hello engine rebuild! also, the fit and finish of the z is superior to the sti imho. close the door in a z, then close it in an sti... 1 of them sounds like an empty beer can and its not the z.
handling better is definately up for debate. the sti's corner entry speeds are much slower, but exits speeds are much quicker. the z u can kinda toss into a corner at higher speed and it will have enough traction to settle down and pull you thru. sti's will understeer like a mofo if thrown into a corner too quickly. however they can exit much quicker because of their awd system.
there is no comparison if u want a sports car. the z is a sports car, not a sports sedan. however, if u need a little utility, then the sti is a great bang for the buck sports sedan. it even performs great on the track. however, steering and driver feel is 10x better in the z. the sti is quicker from the stopligths without a doubt. i mean those things can launch HARD. but the Z is faster in general. meaning that once u are rolling a 300whp z can end up reeling in a 330whp sti without any problems.
really comes down to what you are needing out of a car. if u need a little utility and passenger space, then the sti is for u. if u want a true sports car that will get looks everywhere u go, then the z is your car