Deffboss's WAAAGH!!! energy would put him on the same psychic scale as the Chaos Gods. His strength would be comparable to Khorne, and the Orky fauna living on him would be potent enough to compete with the biohazards of Nurgle.
If this guy gets into the Warp, Chaos is done for. Hell, Malal would probably side with him; since the Gods would almost certainly unite against him, that unity would piss off Malal enough to take action. And Gork and Mork would undoubtedly get involved too, we're talking about the biggest WAAAGH!!! ever, on their turf. The Emperor would like the idea of the Chaos gods dying, so he would help out as well. Cegorach might want in on it for the chance of offing Slannesh, but I doubt it. Warp War I isn't exactly his thing.
Gork, Mork, Deffboss, Malal, and the Emperor against Khorne, Slannesh, Nurgle, and Tzeentch. Place your bets people.