Actually, SL were minnows during Imran's era while Zim was a decent side in 90s. So, that is one. Now In ODIs, when we compare though, Wasim was the greatest ever while Imran was great but nowhere near Waz in that format.
Anyways, all these are stats and numbers which alone are never enough. When you look beyond it, the peer-reputation which Wasim has got is second to none. The biggest legends of the game, the likes of Lara, Viv, Ponting, Kallis, Sangakkara, VVS, Azharuddin all regard them as the best bowler they faced.
Even the likes of Botham and the contemporaries of Wasim, i.e. Donald, Ambrose, McGrath, all of them consider Wasim the most skillful and complete bowler of their era.