Well I had an upRev done with standard airbox + cosworth filter, plenum spacer, HFCs + Milltek exhaust and was well over stock before and 300bhp after
All that on a 2004 Zed on 100,000 miles
They are mostly for noise though, they give a nice throaty tone
I am sure there is a vid of a load of us pulling out of a layby in Wales I will have a look....