If a UTH is getting hot enough to melt plastic, you have more things to worry about than the plastic--like the animals inside the tote. The melting point of plastic is WAY higher than the point that would kill even the hardiest reptile.
Talking about the Zoomed ones, I have personally temp gunned two Zoomed UTHs of two different sizes (5? gal and a 30-40 gal sizes) after letting them heat up for a while and had them both come up in excess of 130-140*F. Boy oh boy did that ease my worry about "wasting" money on thermostats or what? Ha! I also had an All Living Things one reach about 100*F without regulation... Not as bad, but still not acceptable for the corn snake I was going to be using it for (again, thank goodness for thermostats!)
I've had good luck with the Ultratherm UTHs sold by the Bean Farm staying on the cool side, though. I'm using the smallest size under my 20 gal dubia tub right now... I haven't temp gunned it, but the roaches aren't complaining, the plastic isn't melting, and I've never had one of those go over 95*F when unregulated, even on a solid shelf (as opposed to the ample ventilation the one under my roach tub gets thanks to the solid shelf). I still wouldn't (and haven't) use one without a thermostat for a pet, but I'm not too worried about the roaches getting cooked over one! =)
... And only now that that is all typed out do I look at when this thread was posted. Oh well. I've already got it all typed up, may as well post it! Hopefully someone will be able to glean some use from this post.