Hey, I bought a Prospeed for my 172 - I'm not knocking them. I've had a few problems with it, similar to Tim, but it does a job.
My OE exhaust was falling to bits (surprise, surprise) and there was a group buy on here and I got it for £270 fitted. Cheap as chips.
BUT if was serious about performance I'd be looking along a different route than H&S or ProSpeed. Added to the fact the BTB one looks about the best exhaust I've seen yet in terms of quality and aesthetics.
Strangely there isn't a purely performance driven exhaust for the 172 that I can think of apart from maybe YS's offerings. Which were still in development when i got my Prospeed.