How much of a performance gain could one expect from high-comp. pistons? Ive been kinda looking into it for my GT. Also who makes a good piston thats relatively affordable? I did a search but couldn't find what i was looking for.

 

You are looking at basically 10% gain for every point of jump in compression. Probe, Diamond are just a couple of great pistons.

 

Compression drives cylinder pressure, which should be matched to the cam and fuel available. It's simply not worth bumping compression to gain 10 hp when you now have to run 100 octane.

If you have a big cam, you normally need more compression to support it (them). It's all about matching or mismatching parts in your combo.

 

You don't have to run 100 octane to run 11:1 c/r. You can pull some timing out for your tame tune, and then throw it back in on a high-test tune. Higher compression makes better torque, and you could gain as much as 25 hp. You can't possibly add enough timing to make that kind of gain, so the trade-off, at least for me, is well worth it, especially if you have the ability to switch tunes readily.
If you're running nitrous, or big cams as stated, your engine will love it.

I have a set of 1.8 cc pistons (brand new) sitting in my garage that I was saving for a high-compression n/a build, but I bought a 4V short block with the same pistons in it. If you want the new set I have that I no longer need, PM me. They are brand new, Ford OEM pistons. Your c/r would be ~11:1 to 11:5:1 with these pistons.