If a 55 will punch clean through I would rather have the extra speed.
I think you are exactly right. Speed is your friend with mono metals. If a heavier bullet penetrated better, you wouldn't be able to tell....
FWIW I was never able to "catch" a 50 grainer after a dozen kills with it either.
I switched to 55s simply to increase the bearing surface, and hence pressure, and hence velocity. I really wasn't getting the speed out of the 50s that I can get out of a 55...