Thom,
I did stretch a SC20 hull to 23ft once as part of a hull fineness ratio study/test. I put the boat together with a standard 20 hull on the other side. I ran speed tests on this system with speedos many times. I remember that when the boat would go 20 on one tack; I could come about and load the longer hull and the boat would go 24 or 20% faster.
The ARC22 has the bows from the 23ft test hull, finer entry and reduced bow wedge angle. The 22ft length was chosen because by moving the transom forward 2ft one could have a 20ft hull with lower drag the 22 bows all designed for ocean sailing. That boat could be built today out of the 22 molds.
Bill