the 1200 feet per minute piston speed come strictly from experiments ... no theory to it, just many many dyno tests of brake specific fuel consumption (BSFC)
this book has a good graph showing fuel economy by piston speed and manifold pressure near page 378:
repeating for emphasis you can create engines that get maximum fuel economy above 1200 fpm, but only if you use special bore coatings (ceramics), special rings (very thin & hard), etc
or "chop the Gordian Knot instead" and do away with rings altogether