Powerflow Exhaust have become very popular with Grumman AA5 owners in the States. To get some feed back do a search on the Grumman Gang archive site at
Grumman-Gang Mail Archive and put a search in for "Powerflow" I think you will find some interesting reading there.
There is a Service Centre/Dealer for the system in the Midlands, PM me for the contact details as I believe advertising is not allowed here.