Without more than adequate cooling, I'd have to say no, you probably can't reach XT-PE speeds. Heres my personal opinion regarding flashes from PRO to XT-PE. (I've used this philosophy quite a while a go, so don't expect anything up-to-date):
Lets say you do buy the VIVO which is PROBABLY the only one that can flash to XT PE. Last time I checked [(1 month ago) told you this is going to be old] the VIVO was what, $360-$380? Why risk the possibility of a bad flash that might or might not do you any good; when you can put in an extra $20 and be guaranteed without a problem? But that's just me. Also, with technology, the first "batch" that comes out is usually the best.
EX: BFG in the beginning= it was the new rave DUDE!
BFG now= Meh, it's been better.
*VIVO then= Can easily be flashed b/c they weren't locked.
*VIVO now= Probably hard to flash to XT PE b/c ATi got smart!
LEGEND:
*= Old knowledge kicking in...