In our vehicle drawers in the back of our poluce vehicles, the body metal hits 170 at 100 F ambient. Inside the drawer it never gets over 140. It would require outside temps be over 125 F before I would worry.

The dashboard and glass of vehicle interior hit 160 at 100 ambient. Keep armor out of direct sunlight, off your dashboard, and outside the engine compartment.

I'd like to ask the end user with 180 F armor how he intends to keep it on while his skin blisters? 180F water causes 2nd to 3rd degree burns in .1 seconds, 170F water does it in under half a second.

Here in Vegas, outside temps reach 120+ during the summer.  In the trunk, I have observed 165-170*F temps.  In the cabin, with dark tint and front screens, I've seen 140-150*F frequently (every day for a month or so).  I frequently go to Arizona, where I've seen 130+ ambient temps.

I won't have the plates in the trunk or in the car much, but I don't want that to be something that does happen a few times, and then I've got a false sense of security with degraded plates.

Tests sponsored  by Toyobo (Japanese maker of Dyneema brand UHMWPE ) have put 100% UHMWPE hard armor plates in a 170 F oven for 5000 hours and it still stopped multiple hits of 7.62x39 afterward. A few hours a day, a few days a year, at 165-170 would not worry me too much, especially if you have a non degrading ceramic strike face. If you hit 180F I'd not consider trunk storage. For affordable level IV plates, manufacturers  mostly stick to fiberglass and Aramid fibers anyway, dye to price, which are fairly heat insensitive. But if you want 5 pound or less III+ plates, they are likely to have some Spectra brand HB212 (premade layered unidirectional UHMWPE) layered in. Also keep in mind that the ceramic face and rear side padding will insulate the inner layers so the first hour you get 170 on a plate face does not equal the inner fibers hitting 170.

Good info.  So what UHMWPE plates will stop M855A1?

I'm pretty sure you will need a combo Ceramic/UHMWPE plate to stop M855A1.  I think UHMWPE by itself will get punched.

Any thoughts on the lightest plate(s) that will stop M855A1 under, say $500/plate?

The 3S9 is 5.3 lbs in a 10x12 shooters cut.

I don't know anything else lighter than that off the top of my head except I would guess the Guardian RST from Highcom would do it and it's only 2.9 lbs in a 10x12 shooters cut and only $50 over your limit.

I have requested one to test they said probably next year.

Hesco BES 4800 will stop it and weighs 4.9lb in 10x12 or 5.0lb in M SAPI. Pretty much twice your budget though.

The number one factor of how well your armor will perform is how much you train in it.