Playing rumble will get you playing better in the other modes so keep it up. My stats are not great but they're a lot better than they were all down to playing a lot of rumble for the grimoire. It will make your reaction times better, map knowledge, when to pull out of a fight, when to kill and take cover to get your shield back. You pick up more of these things because there's less people, more room to move around the map. When your winning regular rumble matches with double the score of the person in second place you're going in the right direction.