Conventional tuned pipes are great for all out speed but not for 3D applications (Bad Throttle response from idle to mid range).

With the .91 FX, I have personally bought this one:
[link=http://www.jettengineering.com/accy/jettstream.html]Jett .91 Tuned Muffler[/link] Mine broke at the header due to too much vibration tho.

This one will work as well:
[link=http://www.pspec.com/details.asp?ProdID=61&category=8]ultrathrust muffler[/link]