I never had a problem carrying in 115* heat with 46% humidity (heat index of 157*F). You just sweat more. The A/C in our Landcruisers could only keep it down into the 90s inside the vehicles. In the early months of summer the humidity was lower, which just made it feel like we were in an oven and sweat evaporated almost as fast as it was produced which gave us a "crispy" sensation.
IWB with an untucked button down shirt as cover garment and a Mr Davis bamboo compression undershirt. At that temperature the undershirt becomes soaked which helps with evaporative cooling, and also prevents the gun from "sticking" to you in the heat.