The PSE gives you noise, but not a lot in terms of added power or improved fuel economy.
A full sports exhaust is really the only way to be totally satisfied, but if you want to hold onto a Porsche Extended Warranty which is probably wise for all with Gen 1 997s, then it has to be a Porsche System, so probable the PSE is the only way to go, fitted by an OPC to be safe (Warranty implication if question about people who fitted).
While my experience is that you can get "bulk" deals with an Independent, I think you are less likely to be able to do this with an OPC, mainly I believe because of their standardised accounting systems.
But give it a go.
Regards Big Bob