Optional rule. Quite horribly broken IMHO.
Granted I've never used the rule, but why do you call it broken? Reduced heat, YAY. Reduced damage, booo.
The only really abusable point I can see is dialing a Clan PPC down to 12 damage to still have a potential headcapper with slightly less obnoxious heat loads.
On the other hand, Prime versions of Puma, Hellbringer, and Linebacker become a bit more viable. And as Weirdo said, Nova Prime can turn into a running alpha, heat neutral crit seeker, which gets scarier the more I think about it.