The thinner .015 gasket will increase your compression almost 1 full point! I get a .95 increase.
The thing to watch though is your quench height - make sure the distance between the top of the piston crown and the flat area of the head stays at least .030", .035 - .045" is ideal IMO.
The way to check is to measure the deck height with the head off. Just measure the distance between the top of the piston at TDC and the deck. Then add the compressed thickness of your head gasket. Factory deck heights are usually around .025".
If you add a .015 thick gasket to a .025 deck height you get a .040" quech height (perfect). But if your block has been zero decked and the piston is even with the top of the deck .015" brings you way too close and you will probably have a little collision at the end of an exhaust stroke at 6000 RPM! Not good