The actual torque delivered by an impact wrench will vary greatly depending on how solid the connection is. This is the main reason that "impact" sockets are so much heavier than standard sockets; the standard sockets will flex enough to loose a good deal of torque. Getting a connection to a knockoff that is both solid and reliably repeatable is difficult even with the hex knockoffs; impossible with the eared ones.
Personally, I don't like using an impact wrench to tighten even ordinary lug nuts, because of the repeatability issue. I have seen too many professionals that get them either way over tight, or not tight enough; sometimes on the same wheel! If the impact is handy, I'll use it to initially run the nuts up; but final tightening is best done on the ground, with a hand wrench (or better yet, a torque wrench).
The factory engineers clearly knew what a torque wrench was, and revised the procedure for tightening the knockoffs several times. I have no doubt that, if they felt a particular torque was the right way to go, they would have specified it that way. Witness the service bulletin about the special nuts for the splined adapters, which calls not only for a specific torque, but driving the car and then rechecking the nuts.