Horsepower limit for out of the box Edelbrock RPM heads

Since they’re just another copy of the RPM heads, the Speedmaster head would easily support 500hp, used with the correct support equipment.
A decent solid cam, upgrade the springs, etc.

Want a little more?
Do a proper valve job and blend the bowls.
If your combo could tolerate a pretty decent sized cam, get the CR up closer to 11:1, and then the “easy” number(with corrected vj and bowl blend) gets closer to 540-550hp.
But that’s no different than with RPM heads.