The ultimate setup? I would want equal length headers into a high flow metal cat into a titanium exhaust with PSE type switch controlled bypasses. I have seen the
Akrapovic systems for other Porsches, and they are incredible; light, impeccably crafted, powerful and almost musical in sound. Granted, the cost of an Akrapovic system is probably as much or more than most 986s are worth, but you did say you wanted the ultimate.
In my opinion, the sound of the Boxster is one of its best attributes. Anything that can enhance that attribute without drone is a win in my book.