Trail Rider Magazine - http://www.trailrider.com/ - have a quite in depth review of the KTM 520 EXC RFS, comparing it to the KTM 400 EXC RFS.
Seems they found the extra power to be good in the tight going, defying the traditional belief that the 520 is just "too much bike" for the tight woods.
Check it out. Their thoughts may help you choose your next bike.