I've had 11 IF and it was only useful with the fattest armor. Even in regular mail, I couldn't take an additional hit from 2h or pole.
If it was possible to make IF give +3HP the devs would already have done it. Sadly, it's not possible ATM. They can only do whole multiples of its effect, so it's either +2 or +4, which would be a bit much, I think. Maybe they should try it, though, would be easy to revert if it turned out OP.