I've used steel wool right down to 0000? on epoxy. You can get a near mirror finish. Works on varnish too, but I think the epoxy is a little harder.
(basic operating theory is whatever would work with plexiglas acrylic would would on epoxy. So long as any wooling, buffing polishing done on epoxy or varnish doesn't heat and soften it, it'll work.)

However.... based on the now banned super fast competition swim suits which have a micro-texture (based on shark skin I think) I've let my hull go matte. I'm willing to bet that a hull in water sanded no higher than maybe 220 would be more slippery than one buffed to multi-zero steel wool reflecting like a mirror. See the golf ball texture car episode of the Mythbusters if you don't believe my malarkey.

An easy test ( I should do instead of speculating) is to make two hydrodynamic shapes, polish one and leave one at 220 grit, hang them on opposite ends of a stick, drag it behind my kayak and see which one has more drag. Longer shapes--more surface area, would probably yield better results.

In any case when I've had a GPS my Great Auk hull has been a fairly steady 4.3 knots with the largest variable the guy swinging the paddle, the wind and waves.