Rivcos do look nice, and I have a riding buddy with them on his F6b, so I've tried them. For ME, they are not comfortable to use. On my F6b, I have Ergo IIIs.

JGF6B - Interesting point. I've never tried to run mine very far forward, so I've never felt like they're too long. Mine do have multiple holes in the arms to change length.

I keep them adjusted so that if I set my heel on my pegs, my toes are on the Ergos. That way I can ride 1) just on pegs, 2) tip of heel on peg, toes on ergo 3) just on ergo, or 4) achilles on top of ergo for full-stretch. Those small changes seem to help stave off discomfort. Of course, YMMV.