If you get "too hot" the bullets will have a frosty look to them
Contrary to popular belief, it DOESN'T HURT A THING" TO HAVE THEM THAT WAY.
If you need more heat, use it. I would no go much higher than 750 however.
I have found that adding a few very fine lines from the cavity to the outside of the mold to vent air is very helpful.
I use a graver, and vent my Lee molds with about 6 lines. I get excellent bullets that way.

One more thing
If you add 1 part tin to 30 parts lead, the metal will fill out the mold a lot better.