I was unable to get a test ride,on a 5spot I can ride medium or large but the large burner has a well long tt so was not sure if it would be to long.My medium has a 70mm stem with 740mm bars,only had 6 rides but love it,my spot is a heavy build with 36 forks 2.5 tyres,angleset + dropper post,a good lakes, alps bike.
The only rfx I have tested was a 2007-8 & my spot is a better bike!
The burner does take a little bit more effort on the climbs but if you push hard it will keep the speed.
The only other 650 bikes I rode,intense traser @ cube but the burner feels better.