Exactly. I have had steel brake discs glowing red...how hot would that be? Plus, all polymers lose strength at any elevated temp. 500 is just the point where they decided it was still "strong enough". The question is, "strong enough for what"? You can use it if you want...I'm just saying I would not want to ride in a car with the brakes held together with JB Weld. But that's just me.
The entire plate is not that expensive, here is one on Ebay for barely more than a Heli coil kit:
Find many great new & used options and get the best deals for Triumph TR3 Brake Mounting Plate Left side at the best online prices at eBay! Free shipping for many products!
www.ebay.com
I'd just OBO this guy and never worry again.