I am very skeptical there is much of any horsepower to be had below 3,000 rpm from metric rings in a flathead. The Engine Masters show did a back to back dyno comparison where they took a small block, tested it, swapped otherwise identical pistons with metric rings, and tested it again. There was no measurable difference below 4,000 rpm. For members who want the most out of their engine way up in the rev range, then metric rings are an improvement. For members who don't drive that fast, or who like me who install overdrive and simply up-shift, there is little point to spending all the extra money.
Here is the link to the back-to-back standard vs metric ring test video, nice comparison graph is at 8:40 in the video:
https://www.youtube.com/watch?v=-29IWc-zrv8