what kind of hp difference between heads?

What cam do you plan on running?

If you want to realize the .600 lift flow, you'll have to lift the valve there and beyond.

You may want to think more about this.

If Im not mistaken..the rhs heads done up only another 6 cfm @.500 over what you have now..

RHS flow from IMM's site
Valve lift Intake Exhaust w/1 3/4 pipe
.100 69 58
.200 141 119
.300 197 161
.400 241 184
.500 267 193
.550 271 195

In your shoes...I would upgrade to a roller cam before dropping $1400.
But then others will talk closed chambers...and all the other ways to do the same but in a different way.

No matter, either way you wont be disappointed.