its most likely the rings. if you want higher compression pistons (which i would suggest for N/A, usually people go with 12:1) you can just replace the pistons, new pistons usually come with rings. kill 9 birds with 1 shotgun shell.

by the way... what engine do you have? B18B? B18C? because i wouldnt suggest 12:1 pistons on a B18B block without a GSR head.