Here's an important thing to remember about DR, it likes to be applied before DT. This is why veteran rangers are so tough in a stand up fight they have 30% DR ontop of there DT. If you take some Med-X while wearing your power armor, its effects increase dramatically.
I think power armor may be air conditioned, heat would probably be an issue in any environment.
Yes I know, the NCR rangers due to this are equal to deathclaws in strength and armour, yet a power armoured BoS is weaker.
PA on its self is not great, not by a far margin.
Take on MI securitrons with it and don't fight back your health goes down rapidly, due to loads of pin pricks.
I had DT 40+, DR 25 and buffouts boost to health, 3 elite rangers took my health out in 20 seconds while I was trying to kill them.
Just the same as PA in FO3 against the enclave.
Without the DR the securitrons around NV's strip knocked it down to a third, even when I killed them within two blows.
Just the same as FO3 with every one armed with pistol.
Compare this to FO1 and 2, where a PA person would take at most 2 damage from all but the strongest guns.
DT has a use in NV it slows damage down a bit, getting anywhere near the DT to do so effectively is a challange however.
The only way to effectively avoid damage in NV is just not get hit, kill them first.
Which makes PA a joke imo, make it as slow as a snail and then make it impossible to bite through without the right weapons.