The only time so far that my 'faust hasn't killed a Sherman in one shot is when I accidentally hit the gun mantle. It caught fire and the enemy bailed, but the Sherman didn't die right away. I have one shotted a Sherman from the front with the 'faust at least once, though.
The PIAT on the other hand, takes me an average of 3 shots to the side (not hitting skirts) or rear to kill a Panther. I can knock out a PzIVH with one shot to the side (again, the small parts not covered by skirts) or rear with the PIAT, but I haven't knocked one out from the front yet. StUG is the same story as the PzIVH, one shot side or rear, more than three to the front (not actually having been able to kill it with front shots yet either).
So far, the Panzershrek has killed everything in one shot, and I haven't played with the Bazooka enough to notice how many shots it needs (other than one shot to the rear of StUG, PzIVH and one shot anywhere on Hanomags).
Speaking of Hanomags, sometimes when I fire a PIAT at it, it looks like it should hit dead on, but the projectile actually explodes underneath the halftrack, and makes it jump a couple feet in the air, but doesn't seem to actually damage it. Weird... I've mostly experienced it on Falaise Pocket, usually aiming at the halftrack with the second or third hole in the ironsights (the medium and longer distance shots). It has happened with the first hole (close range), as well though.