Yes most players on the game have a 50 or < IBK, I would say up to 90% of the players. The only players who are usually above that are FB, TE and OL, DL.
With knowing this, I have tried many different players with high IBK and have not noticed that much of a difference on returns. IMHO. There could be more, probably RBS, RBF as when they lock up with an offensive player, something has to determine the outcome.
Also for ST, its IBK vs. BSH (as block shed is used in the open field) so that may have something to do with it as well.