There's no clear way to know how much the overall bass population is feeding on pellets vs. other fish, at least to a high degree of accuracy.

You could gut a given fish and see what it has in it's gut, but that's going to vary from fish to fish.

If your bass eat feed all year, then I would feed them in the winter too.

Just a note though....when a bass eats feed, I don't think they necessarily stop eating other fish, they probably just don't eat as much other fish. Plus, the other fish may grow out of predation size as they too will be eating feed.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."