Matt -
After lots of issues with heat and paint - I've opted to go with ol' fashioned steel. The Texas heat caused most plastics to sag - sometimes just a tiny bit - but sag nonetheless. And, no matter what I tried, the whack of a weed-eater would peel the paint off of PVC posts in short order.
I'm learning the ins and outs of powder coating for the posts. The other support sections are primed and painted with a high-temp product - and so far (knock on steel) no rust and no sag. The powder coated posts show some surface markings from the weed-eater - but I was basically trying to chew the post down with it. 
I should note that the draw back to the steel is its inherent conductivity. I'm leaning heavily towards battery operated locos as it eliminates any issues with connectivity between track sections. My biggest fear is that a youngin would get shocked if power was run through the track and they touched it.
BTW - kudos to Norm and his Rail Bender!!! It's built beautifully and it works like a champ - right out to the very ends of the flex track. By permanently bending the track - I no longer have to screw it down - which helps to ward off any problems from expand/contract. I'm also working on a retention spring that keeps the track ends together through the expansion and contraction cycles.
After almost two years - I'm in too deep to quit! LOL!
