(In my quest to keep the twenty-somethings in sight until at least the second turn) Can an epoxy film be polished to as smooth a finish as varnish? Or, can paint be polished smoother? I like the idea of keeping the bottom of the boat epoxy-only, for easy repairs and refinishes, but will give that up if there is a better choice.

Epoxy can be polished to a high gloss if you wanted, however, remember that varnish and paint are not only to provide gloss, but also to provide UV protection. Epoxy does not play well with UV and will deteriorate over time without protection.

If you are very careful with an epoxy-only boat - keeping it under cover when not in use and not using it for extended periods of time in broad daylight, you may be able to get away without a protective coating, but it is likely not going to make maintenance easier. You will need to sand more often and apply more coats of epoxy to replace what you sand off.

If you want low maintenance, go with a high quality paint. It provides the most UV protection and longest lasting finish.

I don't know about the efficiency benefit of a buffed-high-gloss vs fine sanding, vs out-of-the-can-gloss. I think you are talking differences of a boat length over 1000 meters. If you are falling way behind, you need to work on tuning up the motor.