The advantage of the 90 degree-rated hats is that you do not need to worry about exceeding their temperature rating even on a hot summer night with the lights on for an extended period of time. The insulation around a fixture does not prevent heat transfer from the fixture. It slows it down, so that the interior of the fixture runs hotter. However, over time, the area immediately inside of the hat does become heated. The question you are really asking: what is that temperature under the worst conditions? So perhaps you should err on the side of safety to protect yourself from liability. The thermal safety switch in the fixture should prevent ignition of surrounding poly if the poly is rated for a temperature that exceeds that of the thermal.
You have probably already considered that the hats are not necessary in the lower ceiling of a two story house, even when insulated for sound. Vapor transfer is an issue only when moving from a warm space through insulation into unheated space.