closed chamber 714 casting 318 heads

I picked up a set of these heads fresh from the shop for 300 bucks. My old heads needed completely redone. Figured I would save some and get the benefit of the closed chambered heads. First of all, in a perfect world how much power should I expect to gain from these heads? I understand they are swirl port heads, would I see a fuel mileage increase with them? The goal is basically a fresh engine able to run on 87 octane. Stock pistons are .075 in the hole and I will be using just a regular felpro gasket in a master gasket set. Even with the lower compression will I see benefit from just the heads?