true some are on wood, fan is top side so I dont think it makes much of a difference vs the ones directly on metal rails, considering the high ambient temperature...in winter all will be fine.  And they run a bit more than 80%...1000w evga's GQ, solid PSU, running anywhere between 750 and 930w depending if im dual mining or not. (at the wall, 120v, so on 240v its a little less due to better efficiency).  Breakers all run at less than 80% all up to code, but the PSU's themselves dont really need to if they are quality (of course you are correct lifespan with high temp is shortened).  Warranty is 7 years though...