Well, the ITR brakes OWN GSR ones.. so I wont even go there.

These parts on my 2000 GSR put down 169whp/123wtq:
AEM CAI
JDM ITR 4-1 header
Carsound 94006 Cat
no power steering belt
18 degrees timing
VAFC (tuned)

Total, that's what? 1200-1400 or whatever.

I went through a few suspensions and have now settled on OEM ITR springs on OEM GSR shocks. Works fine and is softish still. 22mm comptech rear swaybar is also installed. I've since sold the VAFC and installed the power steering belt, in which condition the car put down 160whp/120wtq.

Tq curve was always pretty much flat, although a little more flat with the VAFC installed but it still has decent midrange.

IMO, if you can get the ITR, do it, if only for the brakes and LSD (that's my biggest complaints against the GSR) and you can live with the slightly elevated noise. Cruise control is a PITA to install but can be done.