I've been fiddling with Gosen Polymaster II as a cross for Pacific Classic and like it so far, but I have not tried Co-Focus as a cross. I went with Polymaster due to its shape, so that its flat profile would lengthen the life of the gut by not sawing into it as readily, but the ribbon shape also of course reduces friction: a full bed of it has a static CoF of 0.074, almost as low as it gets. Low stiffness, too: 172 lb/in (strung at 51 lb, fast swing). It's a little pricey for my taste, not being available in reels, but since I won't be cutting it out before the gut breaks I guess I can live with it.
All that said, I'm not sure how much guidance the friction tables are going to give us for poly crosses to be used with gut. TW Prof has noted in the articles about string friction that the full-bed CoF's are not reliable predictors to the friction between two different strings, even two polys, and the oils released by gut mains introduce another friction-affecting mechanism.
A gut/Hepta-Twist 17 combination does appear on the friction tables, at the very top, and Hepta also has a lower stiffness rating than Co-Focus, so that seems worth looking into. Seems like the Hepta would shred the gut, though, doesn't it?
I don't know what to think about nylon-based synthetics that have lower CoF than some polys. It does seem worth trying with gut, but it's an expensive experiment!
In my youth I was addicted to The Hokey Pokey, but I turned myself around.