Grebby and Ork commander make those orks really freaking scary. Imba? Not so sure. You can still tidal the EXGO, isolate Grebby, whack him with WK, volfied, the usual perpetrators to counter orks. But having something akin to a kaldrao+exlap buff on every ork sans DF can get very scary very quickly, especially combined with his revive skill.
I suppose I'm generally against snowballing vitality/df since with agility/range/attack, you can still kill something with an open skill/SS, and maybe some attacks. But if you buff the HP+DF of something beyond SS range that wasn't supposed to be (especially a whole group of somethings), you can get from fair to broken in short order.