Better than Hadlee? Don't think so
I'd say Imran in the early-to-mid-80s was in contention for #1 bowler with Marshall and Hadlee.
Imran's bowling career was a bit different than Hadlee's though they both started in similar fashion : ordinary bowlers who developed into tremendous ones.
Yet, they differed in one significant aspect : while Hadlee started as a fast and erratic tearaway, Imran started as a medium pacer with the inswinger as his stock ball.
Imran scaled impressive heights by cranking up his speed while Hadlee did precisely the same by doing precisely the opposite.
Hadlee however, once 'matured', started producing consistent bowling displays day-in-day-out like McGrath did and he maintained that till the end of his career.
Imran, once 'matured', turned in 4-5 years of absolutely stunning bowling performances before declining to a 'very very good' standard.
In the early 80-85/86 period, he was, IMO, second only to Marshall. After 86, Imran was a different bowler. He had one fatal flaw in his bowling that ended his peak : his trademark 'giant' leap right before delivery. This put too much pressure on his leading ankle and predictably, it collapsed under stress fractures. He came back with a less jumpy action but could not recapture his 'best' since he lost considerable speed.
All three mind you, had their specific and unique 'advantages' :
Marshall had the luxury of bowling in the best bowling lineup of alltime for most of his career. He started with Holding (and arguably, Marshall-Holding opening combo is the most devastating in cricket's history), Garner, Croft and Roberts and finished with Walsh,Ambrose and Bishop.
Imran had the advantage of favourable umpiring. I know that home umpiring always had to contend with allegations of bais - real or perceived but in cricketing circles, it is also common knowledge that the most biassed umpires were either Aussies or Pakistanis of that era.
Hadlee's advantage was the Kiwi tactic of 'lets-prepare-the-greenest-pitch-we-can-find-and-wait-for-Paddles-to-nuke 'em'.
This is perhaps why i'd say Martin Crowe's record is not as good as it should be : I don't consider him to be in the same echelon as Lara-Tendulkar-Viv-Gavaskar but he should atleast be considered as good as Lloyd, Kanhai, Harvey etc. group
But all in all, i'd say that one big reason why i'd rate Waqar, Donald,Imran, Lillee and Bishop ultimately below the likes of Akram,Marshall, McGrath,Ambrose,Hadlee, etc. is because once the former group lost their pace/bounce, they declined noticably while the other group did not/had very little reliance on speed.
Akram,McGrath and Hadlee were not reliant on it. Marshall and Ambrose became excellent fast-medium bowlers near the end of their careers.
Ambrose though became largely a containing bowler in the last year and half of his career because his knees were bothering him so much that he bowled in the mid 70s mph range.