Building a stock stroke Eddy headed 440 w/LY rods for a customer using above mentioned SRP pistons. They are about .020 below the deck and with the current Fel Pro gasket compressed thickness of .039 I'm coming up with a compression ratio of about 9.74:1. Would going to a Cometic with .028 compressed thickness be a very noticeable improvement and worth the trouble? Customers car is a 74 Cuda with Edelbrock Projection fuel injection. Nice street car.

Very little difference.
Different animal but we went from 55 to 28 gasket on my smallblock, and cut the heads 50 thou and both these iteams moved compression up 1 point.
What your considering doing is probably a 1/10 of a point.

Which heads are you using on this motor? If iron stay well below 10.00 to 1 comp. ratio

020 + 028 = 048
020 + 039 = 059

Pretty much quench vs no quench. Does it need quench? Probably not, but you could perhaps run a lower octane fuel with the quench.

I'm installing the 84cc Edelbrock Performer Heads. I'm wondering how much performance would be gained if I have the heads milled down to perhaps 80cc?

Why not just cut the block .020 so its close
or at zero then use the heads with the .039
gaskets

I'm just having the Edelbrock heads milled to a 75cc chamber. This should yield me around 10.56:1.