To me the fun part about DW is facing down truly astronomical amounts of damage and still ticking after. Great cover and high miss chance is not going to keep the players on edge about their survivability. If you make orks pretty incompetent at combat it's generally going to be a boring experience for the players fighting them and for you as GM running them.
My suggestion is to have a few "named solos" equal to the KT's number - probably Nobs or even Meganobs, or add in a Weirdboy if you want. Run the plain old Hordes as limitless (as in, they rejuvenate all damage done to them at the turn end, so reducing their Mag does help by limiting the shots they can take and bonus damage but does not get the battle closer to resolution since the hordes will not be destroyed conventionally). Force the players to leave their cover to kill the horde leadership and feel free to reduce Horde mag whenever, say, a Meganob dies as a group of orks breaks and runs since their commander got killed. When all the named solos are dead, all remaining orks break and run, leaderless.
Oh and don't waste time rolling for NPC vs NPC combats - i.e. allied hordes vs enemy hordes. Those volleys should be determined by fiat based on the needs of the battle/plot, i.e. in some places they can turn back the rush, but let IG forces be overwhelmed so the players have to make the choice between holding their area or reinforcing the IG's position. If they don't reinforce, have enemies coming at them from multiple sides.
Yeah we're almost done with the mission, maybe, at the very least there is one more wave to show up after reinforcements, we did lose one of our squad, but we weren't too upset because he was going insane very quickly, he got blown up by artillery basically and he was in the middle of a horde.
But there are going to be 7 nobs by the last wave, and the map is pretty large and they have just started getting into combat, one is at half health (I'm using regular nobs and mega nobs, I have the entries in the Rogue Trader bestiary that I am using), and the others are close to my players. But they will probably be a challenge for the two assault marines, a Black Templar and Carcharodon, the latter is also frenzied. I'm pretty sure the Mega Nobs will give them trouble, one is on the map and 3 more and a Kaptin are coming in the next wave too.
Next mission will be Noise Marines, a lot less hordes too, one of my players, the one who died, told me he isn't a big fan of fighting hordes. I think I'm going to be pitting my players against enemies who are more challenging one on one soon. I've already made missions involving Eldar Exarchs (using Rogue Trader entries from Koronus Bestiary), Noise Marines, Berserkers, and a big battle with tyranids.
I'm going to draw up a mission with Necrons, and another Tau encounter, I think Fire Warrior and Necron Warrior hordes are the most threatening compared to others, since they have good armor and ranged firepower. We've only taken 3 wounds over the course of 2 missions and I plan to change that.
Its definitely a learning process though.