http://www.hotrod.com/articles/dyno-tested-thin-piston-rings-worth-horsepower/
Over the past weekend, I watched some of those Engine Master vids. They a 5/64 vs. 1.5 ring comparisons. The same exact piston in a SBC with different ring packages. The torque required to rotate the shortblock was vastly different. 37ft vs 14ft for the 1.5 mm rings. That's a huge difference. But the dyno don't lie. I think it was worth 7hp and 4 ft lbs. On a 3.7 bore the difference would be much less. On a 4.5 bore, much more.
I think it is not enough of a difference to justify spending money to get custom pistons. If your pistons aren't hurt, then new rings, gapped appropriately. When I slap that junkyard 351 in I think I am running the cast pistons, but may spring for the Total Seal gapless rings. If I put that BD11 on it, it won't make more than 6psi, so that should work for me.