You won't notice any gains on these cars without an UpRev (remap).
My old Zed (feels weird saying that as I just sold it) after an UpRev made 294Bhp@fly with Berks HFC's (High Flow Cats), Invidia Gemini exhaust system, H.Dev plenum spacer and a Stillen air induction kit.
Standard it should have made 276Bhp so I was happy with the above especially as it was 11 years old at the time so bound to have lost some HP over the years.
It's not all about gains though ~ after I had the Invidia exhaust fitted it transformed the sound of the car as before that it was all standard. Basically unleashed the true V6 sound and made every journey a pleasure especially above 3.5K rpm where it really came alive.