I have just got a special custom version of the OTR pipe for my rally bike (see thread:
http://www.xt660.com/showthread.php?t=10925&page=6) which is approx 100mm shorter than their standard one, and no cat.
Because it is shorter, it tucks up very neatly against the frame, and will mount directly to the subframe, although this may not work with the standard rear tail-light unit of course.
It is very well made, however it is also VERY loud, so I will be adding the dB killer - for scrutineering at least x