Coming to Amir, what he is doing now is above average staff for a left-arm pacer - that's slanting his stock ball across right handed batsmen at pace & fairly accurately, which in tv looks impressive because of the angle, it looks moving lot. But, top order batsmen'll be able to leave lot of his balls on length & line. Even against leftis, he is just bowling out swing that they can leave while his stock ball can easily be negotiated by stressing front pad out side line (by lefties).
The best left armers could do some thing more - they (only 2 in history, may be Gilmore for few maches as well) used to twist their body & wrist in a way that their stock ball would come into right-handersin air, or at least held it's line & then move away after pitching. The best of the best I have seen was the bowled of Allen Lamb in World Cup final, a perfect unplayable ball - if you are too good, one can touch it for a catch, Lamb got squired up & bowled, a tail-ender would have been LBW, for losing balance & caught plumb in front.
I think, Amir is too concern about his front foot, therefore he is not stressing it to the limit, while his front arm & head position isn't up right at release point - once he gets these 3 things right, he'll add more pace in air & movement on & off the pitch. If he can keep his fitness for next 10 years, he should cross Wasim's Test numbers, at least Waquar's number for sure.