If I was you I'd go 19/55, the super speed set. That mill and pipe will drive it easily. The top end on the x-can is pretty good. Also check the x-ports, I believe the short one is for top end and the long one for torque. Might be worth going to a slightly longer one if you go for those gears. You should be hitting nearly 55 with those gears. Check this out...
http://www.rcuniverse.com/forum/fb.asp?m=7172298
Here's my vid of my RTR with stock 23cc engine and mielke pipe, driving the 18/55 gears happily. With your engine and the x-can, you'll be fine...
http://www.youtube.com/watch?v=mcCA4WsRV7g