I would get the JP-4. The JP-4 definitely works well on the LRP, AND it works well with some 21's - at least lots of people run it on 21's.
Check this video:
http://www.youtube.com/watch?v=bZ6OW...A9DD3FDC507F1B
Not a truggy I know, but you can see the LRP with JP4 in action..
If you don't want the JP-4, I would get the JP-2. Though again, the JP-4 is probably more useable with other engines later.. The JP-2 is the best pipe for the LRP I know of for all-out power...