In the game I play, a fellow player is playing a Psyker and utilizing sustained powers to what I find is an unfair extent.

The player argues that because Iron Arm and Warp Speed are free action sustains, he can Push activate both at the start of a mission and have both active the entire time, so long as he doesn't go unconscious (at the Push level for +3 Psy Rating... the whole time...).

Somebody please point out to me the flaw in this setup. This player has begun overshadowing everybody save the Ratling Longshooter who can rival his damage and the Techpriest (me) who can rival his survivability. He's a rules lawyer-y sort and I don't want to start a big argument, so I'm hoping to find something clear-cut in the rules that should prevent this sort of passive power.

At that point, the GM should really bring down the hammer. Mechanically/by RAW, there is not much in the way of stopping this player, but if he is disturbing game balance (and boy, is he), the GM has every right to drag his stats from the upper stratosphere. There are a number of off-the-book methods to "deal with" such a player, but that may not be what you are looking for.

Each sustained Power diminishes your Psy Rating by 1, at least. But yes, the guy needs kicked into touch by the GM really.

Big psyker fan, and not so much of this. Sort of seems to further pull the teeth out of the risk of Pushing if sustaining such power levels doesn't ALSO risk like Pushing did. Hmm... Yes, his Psy Rating should be a bit less, so where about is his Rating? Pushing makes this little loss of Power a bit less balancing. I'm guessing he must be at at least PR 5, as Warp Speed is not for the weak of will, and if you see PR 5-6 as the typical upper limit of Human expectations, some shenanigans should've also been expected. Since each power is being Pushed, however, that's two rolls he'd have to make, each boosted by Pushing, on the PP table; here's hoping one of those rolls blows up in his face. Come on Murphy's Law!