Metric rings compared to inch-fractional rings in this back to back dyno test of a small block chevy found 8 hp at 6,000 rpm. From the graph near the end of the video it looks to me like almost 0 hp difference below 4,000 rpm. I find it interesting they found the same torque difference required to turn the crank during engine assembly being reported in this thread.
https://www.youtube.com/watch?v=-29IWc-zrv8
In the rpm range where I street drive a smaller bore flathead, I'll never feel the difference. Cast pistons and standard cast iron rings for me.