For those that have installed the turbo inlet for STI and WRX. do you notice any real difference in performance/sound. Also for those that used the 3' inlet, did you have to make any changes to turbo/intake?
Depends on the size of the turbo, but generally no. Up to a 30r or equivalently sized turbo the difference won't be very noticeable. The bigger you go the greater the difference will become. Plus most larger turbos will come with anti surge ports these days, which will require a larger inlet to accommodate. My GTX3576r only has a 3" opening at the compressor inlet, but with the anti surge ports the outer diameter requires a 4" coupler to the intake.