I run rubbers on my MTX-3.
I prefer rubbers because they last longer and you don't have to worry about overdrive, underdrive, chunking, and constantly changing ride heights.
Many people say racing with rubber sucks because they are constantly searching for a tire/compound that works, but at our track people use and stick to only three types of tires(Sorex 36, 40, Yok 138G series) so we know what works and what doesn't.
As for performance, several racers(including myself
) are very competitive using rubbers in foam class. This could be because of our track size and design though.