The thinner the ring pack the less drag on the cylinder walls, less friction = more power
On your deal you need to make sure the pistons you buy are for street NOS, not stock or SS racing where the ring lands are raised way up to increase the compression ratio
Are you planning on running a vacume pump? If so then look at the really thin racing rings like 1Mm or the .043 rings with minimal tension Those size rings make a huge difference on the ring drag