Well, ask your GM not to use highly difficult encounters, for one thing.
Mid-level Deathwatch is high-level Black Crusade, to put it mildly. Deathwatch is a game for all space marine groups of significantly higher power than their BC equivalents.
And capping the Orks to RT style progression is not only undermining one of the biggest strengths BC characters have, but also holding them back to the RT levels of power, which is behind even BC by a small margin. I mean, how does he define "earlier than the would have"?
To me, it seems he's using excessively difficult challenges comboed with a somewhat overly harsh restriction on the Orcs in your group.
So just talk to him. Tell him you feel the game is excessively difficult and ask him to scale it down. You're all adults, so just discuss your problems.
But if for some reason everyone want to try and play it like it's about winning, here's a mean old trick for a psyker or sorcerer to pull. And I do mean mean. This is full on rules lawyering dirty, while inarguably utterly legal.
Combo Abhorrent ward and Host of Fiends. An xp investment of 500, so rather affordable.
Push both and put them up. If possible, find any way to get fear ratings in play, for both you and your allies. Have the Orc's go commando, use infamy to get a mutation, use a psychic power. Whatever.
Now, any time someone fails a willpower save within 20 or so meters of your psyker, they get 1d5 points of Corruption PER DEGREE OF FAILURE. They also get 2x the psy rating of Host of Fiends as a penalty to all their willpower tests.
Any time they try and shoot your psyker, they must suceed at a willpower test at -5x the psy rating of Abhorrent Ward. Which, obviously, gets the Host of Fiends penalty as well.
Fear tests are also willpower based.
now note, as soon as any minor NPC (debatory term, but anyone without a name is a good baseline, so most enemies you face) gets 10, they are instantly killed, automatically, and turned into a harmless chaos spawn. PG 289. This is not listed as an optional rule.
Obviously, at psy rating 4 this is good. Pushed to 7, which is easily done, that's 6 each. That's a penalty of -42 on the abhorrent ward test.
Add in anything and everything that provokes a willpower test, preferably with a penalty. Use Force weapons, hallucinogen grenades, anything you can get. With that, even npc's that get to go to a 100 can be brought down pretty quick. Not to mention that having violent mutations happen to you is bound to be problematic, unless he rules that it happens instantly without pain and that they instantly adapt, but that's a bit of a stretch for the logic.