I've chocked a lot of Brackett filters with ice and rain, they are not a cure all, that's why there is alternate air or carb heat.
Rob, you'll probably spend around 1000 bucks to put the round filter on if you buy all the parts new off the shelf. Is that worth a little more static thrust? You'll have to decide that for yourself. If your building or rebuilding form scratch, round is the way to go.
In testing, the round filter equalled the square filter when the element was removed from the square filter. Interestingly, removing the round paper filter element actually decreased thrust (just a little). It seems that the paper round filter is breathing as good or better than wide open. If you already have the round filter, I doubt the K&N would do any better, but you never know till you try.