a thicker one will be good if you want to bring the compression ratio down,but why use such high comp piston just to bring it down with a 1.8mm gasket?
id seriously recommend some rods, for an extra 600ish youll be safe, as now ull have a 350-400kw engine with stock rods, i dont see it lasting, no matter what rpm you keep it too,its rather cheap piece of mind in my opinion.
I have a cefiro, and an ever changing RB30 build started off similar to yours, now its ending up similar to some of the other active users on here.just the addiction.
I dont know much, ive got most/all of my info off here, so just ask away if you have any Q's and cant find em