Having run 17.5:1 compression with a modded pump and 35 psi, I can tell you that it's intolerably sluggish below 2500 rpm and 20 psi. Above those numbers it's almost as good as a stock compression engine running 20 psi. I've read 18:1 is the magic number. Having also tried 16.5:1, and feeling the improvement in gaining 1 compression point to 17.5:1, half that difference again to get 18:1 isn't going to improve performance enough to make it work. There is roughly half a compression point difference between each headgasket thickness.
My next engine is being built to set factory compression in the 1 notch head gasket range with the lowest piston projection stated in the bentley-provided range for that gasket thickness. Then, if my EGT's indicate too high, I'll have the ability to run 22.5:1 with a two notch, or 22:1 with a 3 notch. I have a strong feeling that less than this is too little. Ceramic coating the combustion chambers, valves, exhaust ports, manifold and turbo would help the engine tolerate high temps as well.
my 2 cents.