You're nitpicking my friend, just because he was destructive against the lower order, he wasn't good against the top? Check how many wickets Johnson picked up against the tailenders in the recent Ashes. So that should go against him? No, because tail runs more often than not decide a test match.
Furthermore, Marshall and Mcgrath played in ATG teams and had superb fielding, never mind the consistent scoreboard pressure those great batting sides used to put up. They also played on more bowling friendly pitches.
On the other hand, Wasim constantly had to deal with batting collapses, poor fielding and SC pitches. Mcgrath and Marshall also played more games in their prime simply because Pakistan didn't play and still don't play a lot of tests. Take out Wasim's last 8 tests where he was clearly done and dusted, he averages 22 in 96 tests at 400 wickets.
Swap Wasim into the ATG Australian or Windies team and he'd average 20 or under, I've no doubt because of the freedom he'd be able to bowl with.