My idea from a similar thread was for the benefit of IF to be +1 health, +1 armor. less overall health, but makes ironflesh characters harder to kill. The overall health pool would not be as high, making blunt and pierce weapons more effective, while making cut weapons and other "weak" weapons bounce off more and make the strength build more tankish against low level characters.
it never made sense to me why IF only gave health and not armor; the character is meant to be tougher to kill, and i think this sort of bonus would be the buff that IF needs. perhaps every point of IF would add a set amount of weight, say .2 or .3, so that the player would have some balance to the armor offset; player will be slower and have less wpf than other characters. most people only invest 4-6 IF (I max mine), so the benefits of it would be good for hybrid or balanced builds, but only someone devoted to strength will really reap the full benefits.