I did the corsa track exhaust and went from a trap of 122mph to 125.6mph, consistant with a 30rwhp increase. Heat reduction was at least 80% in the cabin & I have the stock headers and cats, it's amazingly cooler now, heat is no llonger an issue for my wife.
With my RT/10 and it's rear exiting exhaust I did a total belanger & it sounded awesome. With the SRT/10 vert and the exhaust esit now being about 30 inches from my ear, the corsa track at WOT is about as loud as I'd want this car, side exhaust is neat, but to me it's a limiting factor on the loudness that can be tolerated comfortably in the cabin with the top down - which is the only way I drive it, unless at the track.