It's not really an 'Item of Power' in that he can always pull out a spare if one gets damaged/lost. It might be reasonable to take Human Form at +2, however, because while making the change is not rare or involuntary, it does take an exceptionally long time (compared to the default of a supplemental action) and is not possible if he doesn't have access to the suit.
I think that the opposite should happen in that there shouldn't be any Human Form at all. Human Form is made to help offset the fact that shape shifters can't use the majority of their powers in human form and your Item of Power discount should already cover the fact that you can't always have the suit on. Remember that most people with Items of Power can only physically use them for certain purposes, since Stark has no such limitation the fact that he has to take time to put it on and take it off makes up for this. The discount from Item of Power is dependent on how obvious it is to detect so the discount (+1 or +2) depends on your perceptive. On one hand when Stark is actually using the suit is is incredibly obvious, but if you are transporting in a suitcase than it will be almost indistinguishable from a normal suitcase so it's up to debate on what the discount should be. In my opinion +2 is fine, but I can see the argument for a lesser discount. I also think that Mythic toughness is too powerful. Mythic toughness is territory only tromped by beings like Ferrovax and Superman, I think you can get away with the armor but six extra physical stress just, again, seems like too much. I think the "It is what is is" part of Items of Power covers the armor, just make it an armor 3 and add the "No pain no gain" stunt to get an extra mild consequence when wearing the suit. I would also reword the catch a bit too, but I understand your concept. And remember that catches don't stack (except for Physical Immunity) so even though it is common and easy to research you would still only get a +2 catch.
I know it sounds like I have nothing but criticisms but I really do like the character overall and most of my "complaints" are personal opinion and am fully willing to concede many of them to other arguments. On I side note I wish there was another step in between Supernatural and Mythic powers for situations exactly like this. Stark's suit is fully capable of deflecting most (if not all) hand held weapons with no difficulty (I haven't read hardly any Iron Man comics but I imagine well placed fifty caliber shots doing at least some damage), but the fact that Mythic Toughness is practically reserved for the supernatural heavyweights makes me hesitant of letting any PC get hold of them. I especially like your stunts replacing the fighting trappings with Scholarship and Craftsmanship, it makes sense for story wise and saves valuable skill points. I think that making the suit an Item of Power was clever as it is essentially indestructible as Stark can just get a new one the next game (if not sooner depending on where he is).
