Skipping the ballance job isn't a great idea in my opinion, the factory ballance job wasn't that great, and changing pistons can't help the situation. I'd save a few more dollars and put the light pistons in, along with a replacement I beam rod, and take weight out of the rotating assy.
You are right on all counts, and that is exactly where my son and I went on his 340 build: KB243's, SCAT rods, rebalance. It is a $$ decision here, I think. And the OP is keeping the revs under 6k, which helps.
