I was taught in the army to always try and NOT leave your rounds out in direct sunlight for longer than possible. Try and keep them at least shaded even if it is only with a hat. If it was good enough for them then it was good enough for me.

I believe some powders are more susceptable to heat than others. Marketing tells me ADI powders less than others.

NEVER leave rounds on a dashboard during daylight.

EDIT: Dead is dead in my book. Another 100-200 fps is not going to make any difference.