This is something I've been looking onto over the last few months since I got my 996 and I'm still trying to decide....!
Firstly is it out necessity you are replacing? If so the standard exhaust is good for the money at £300.
However if you are looking to uprate, performance wise there doesn't seem to be much in it.
Genuine PSE is silly money and its mostly for the convenience of a switch to flick to change the exhaust note rather than any practical value for money tweak I can see.
Then there's a road race spec exhaust which you can buy as a kit or get fabricated - but unless you are racing and like being deaf, it's probably a step too far.
And there's the rub - the standard exhaust setup is pretty good, and getting beyond it is not really the point - there would be much scratching of the head at this concept by Porsche design engineers who would tell you they tried to make it as good as they could in the first place!
BUT it's widely considered that the 996 in particular has an exhaust note that's , well....lame.
And so there is the Gundo Hack - which you can have done by a fabricator to the the standard exhaust - no extra horses but a much better exhaust note can be yours by modifying the standard exhaust for just a couple of hundred English pounds.
Links are here (inc sound and vision on Youtube):
http://www.pistonheads.com/gassing/topic.asp?t=932619
https://www.youtube.com/watch?v=MEE80_y7FIw
May be worth looking at induction as well to get a few extra few horses; cold air induction pipe (http://www.ebay.co.uk/itm/RAMAIR-SFS-Silicone-Induction-Cold-Air-Intake-Hose-Porsche-997-996-C2-C4-3-6-/390764024791?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item5afb59d3d7) and K&N sports air filter (http://www.pistonheads.com/gassing/topic.asp?t=1006632) will make it feel peppy and sound a bit more raspy/grunty. All this DIY for a couple of hundred.
The above said the stainless option you mentioned sounds good at £515 - where's that from? Link would be appreciated. If it use standard form muffler boxes it might well also take the Gundo hack if you want to add it later - that potentially sounds like a sweet set up for £800 ish!
Let me know how you get on!
J