I think the pistons or rods have to be modified to fit them onto LS rods, but it should work just like a higher compression piston. You'll see your most gain with some tuning and you'll require higher octane fuel, but it will help. I would probably wait till I purchased the GS-R head before replacing the pistons (tearing everything apart once is easier) and the LS nad GS-R heads have different sized combustion chambers, so your compression is going to change (GS-R is smaller, so it's going to go up).

WIth a Type R tranny (which I believe are only available with an LSD, and you want one, probably get more benefit from that than you will higher compression in an LS) and lightweight flywheel and a strong clutch, your car will loose some milage and turn higher rpms on the highway but still be streetable (assuming you don't go nuts with a race clutch).