If its done very carefully and in a tightly controlled temp regime then there should not be any problems. The issue is that for heat treatment of some Al alloys a very careful heating and cooling regime in the order of 180-250 celcius has been carried out, this is tightly controlled and certificated. With powder coating temps vary between 120 -200, it is often not tightly controlled and certificated, some grunts on a line think higher temps for longer is better, and so you can see there can be some crossover between the careful accurate heat treatment and the less careful not so accurate powder coating and there is the potential for f*ck ups. It’s worth knowing the risk, I do not like the idea of handing my frame to a person who may be ignorant of the effects of heat on my baby and has his hand on the lever whilst he is readint eh sun/sleeping/w*nking.
Personally I go for painting, there is an excellent aircraft sprayers near me in devon I can put you in touch with, did a great job on my bullit and spare rear triangle for 60 quid.