Ironflesh 4 is 2 more in AGI, Ironflesh 7 is 3 more in AGI/STR and 1 more in ATH, WM or PS. I don't think it's worth it, better convert to STR and up PS.
If I'm going to do a 2h build again, I'm going for 2X/18, X meaning as high as I can get (but at least 21). WM 6 and ATH 6 are enough IMO. With a good 2h sword, 6 or 7 PS might make the difference between 2 or 3 hits to kill. It's certainly close to the barrier.