LHS recommended I move to 25% or 30% fuel. They said if I went to higher nitro mix I can run richer at cooler temps and keep the power; is this true?

Right now I am using O Donnell 20%.

Last night when I saw my spit sizzle off the heat sink I got scared. So I turned it down 1/8 turn, I'll find some time today and run it again.

Also LHS told me if my engine reaches 240 degrees just momentarily my motor will never be the same again; any truth to this?