i think +2 armour would be enough
Ya I was thinking about the same.  I mean +3 seems too much perhaps.  No less than +1.5 though I guess.  At +2 I think people would definately put points into IF.  
But how would the +armor modifier work you figure?  Considering current armor spread across chest, legs, head - like add the bonus to each I assume?  It would allow people who run helmless to actually have a +armor to that region.  A real bonus to an archer keeping a high wpf, or a speed demon spam monster.
6 pts in IF would then give say +12 armor to each core region.  That's a substantial bonus and also given new maintenance costs allows chain to become plate in essence.  How that equates vs a hp bonus I dunno.