Aluminum heads with a low compression 440 will loose compression? Huh?

I am hearing this, what I call horseshit, thing about bolting on a smaller chamber aluminum head will loose compression due to less heat in the chamber.

Wouldn't less heat allow for hotter tuning? More advance? Ect?

The argument is this "I want to build a "proper bottom end out of my 400 into a 451 to replace my low comp 440."

I say get aluminum heads for the 440, build the 400/451 afterwards. That way you can PROPERLY build the bottom end with the heads you want to use later.

Dudes worried he's gonna loose power/compression that way.

This low heat stuff is news to me, but I've been out the game awhile.

Any decent info on all this?