If you haven't purchased them already, i'd recommend the 420's- with single's you want a longer length can as the engine relies on big pulses of power from the single to be dissipated- too short an exhaust and you loose power.

I've an interesting point of reference on this- if you check some of my other posts I had a custom set of exhausts made for my xtx, small diameter twin stacked end cans, running about 380mm in length.

I've recently put the standards back on-partly because I'm considering selling it. I can't believe how much better the bike is now. Lessons I've learned is 1) the XTX has the exhausts either side for a reason, the handling and balance get disturbed when it's all one side more than most bikes. 2) My exhausts were VERY loud, and that disguised the drop in perfomance I'd actually experienced, despite the new system weighing about half as much as one of the stock cans-this bike actually deals better with the extra weight.

I've still decided to run the stage 1 & 2 air filters and p/c v. If I was starting out modding the XTX again, these are the only mods I'd get

Phew long reply