I did a test a while ago after speaking with the Tech from Hodgdon.
When going from 30 - 100 degrees, here were my findings:
1. Hodgdon extreme powders. 25-50 FPS increase.
2. RE7: 75 FPS increase.
3. H335: 100 FPS increase.
4. Xterminator: 150 FPS increase.
Neither. Know your rifle/chamber and what it does with a certain load.
If you have a load that was developed in cool temperatures that lets say shows MINOR pressure signs at 40 or 50 degrees, don't expect it to show MINOR pressure signs at 90 or 100 degrees....it won't...it may show major pressure signs.
Back off any "hot" loads that were developed when ambient temperatures are cool....pressure WILL go up as ambient temperature increases.
Primers are the key in cold conditions. Bofors powders (Norma and Reloader) are extremely temperature stable. (tested between -50C and +80C). Not enough variatioon to worry about trajectrory to 300m.
For a heat test...remember to warm the rifle as well as the ammo. Where I hunt, the metal of the rifle is often too hot to touch before you shoot and so is the round in the chamber! Gives funny results with some powders and narry a hitch with others...