When Nancy and I got our new leathers a year or two ago -- we ordered them with hot weather and bright sunlight in mind. They're white/silver/light blue -- and way cooler than the dark ones we used to have.
I'm on a M/C committee or two. I have frequently mentioned to racers that - when they are shopping for new leathers - they should consider getting the new suit in light, sun-reflecting colors. Some other things that sure help are two-piece leathers, so the rider can don the pants when five or six vehicles from the start, and only put the jacket on later -- when the run is more imminent. Also - I wear a Kevlar long-sleeve undershirt. It's got a coarse weave that's great for wicking moisture towards the breeze -- I stay quite a bit cooler when wearing it, both without the jacket on and even when I'm fully suited-up. I wear spandex bicycle-rider's pants under the leathers. While there are some that might giggle when I walk around in the spandex (Damn, I know this line is going to show up quoted by one of you smart-butts!), I can move around freely in the leathers and stay much more comfortable than I would be if my sweat-soaked legs were directly in contact with the lining of the leathers.
Nancy and I (and my crew) make sure that whomever of us is soon to make a run stays well-hydrated -- we carry a 2 1/2 gallon jug of sport drink-on-ice and darn near force the rider-to-be to drink of it. I learned the need to have someone "force" me to drink at a riding school one time -- the track steward, about to send me out for a few laps at Road America -- reminded me how hot it was, and asked if I had had enough to drink. I said yes -- but about two laps later was upside down in the gravel from having gone too hot into a corner because I was woozy. I now have others INSIST that I drink something while waiting in line.
My leathers meet both SCTA and ECTA requirements for perforations, armor, etc. I can move around freely and don't feel restricted by "bulk" or "too much armor" -- for sure I'm less restricted that I would be by a body cast and lots of bandages and skin grafts. As for steam burns -- well, I do what I can to minimise the risks, such as donning the jacket at the last minute so I don't sweat inside the leathers so much. You do what you can. . .