I have an Xforce 3" turbo back on my V6 STI.
Mine has the venturi/ wastegate splitter in the dump pipe and the (wanky) angled (think N1) rear cannon.
Boost builds about 200-300 rpm earlier than the stock system. The real performance gain is crisper on it - off it - on it again responsivness, felt most when your on boost, then lift off mid corner and stand on it exiting the corner, the power feels very punchy.
High rpm power doesn't drop off as abruptly as the stock system either.
Logging the boost pressure shows it peaking briefly at 18.5 psi (@ midrange rpm) then tapering to 16.5 by redline, I believe the stock system stiffles the hi rpm boost more, to around 15 psi @ redline.
Fob's STI felt smoother than my car on boost (+ the
ECU now knows what it's doing after an AFM change), but I'm on modded suspension, stiffer sway bars, 17's shod with low profile RS 2's and have all urethane bushes so my car inherently feels rougher.
My 3" doesn't drone or resonate through the car, but I'm still thinking of changing the rear can for something a little more neighbor friendly.
So from my point of view: Get a decent dump pipe for crisper throttle response and less boost taper, 3" mid pipe with a resonator (to keep drone at bay), 3" under diff to an effective but free flowing end can (avoid cannons!).
I've run 3" systems on all 4 of my subies thus far, the change it makes will suprise you in a good way.
Your welcome to have a drive anytime you like, just Pm me to arrange it.
HTH