I’d say Lillee’s extraordinary WPM would make up for being a little less tested than Ambrose. Ambrose didn’t exactly master any conditions foreign to Lillee, since pitches in WI were pretty pace conducive during the time of both. And Lillee faced good competition during his peak, better than Ambrose during his peak. However overall Ambrose faced better competition than Lillee, but not the level Marshall or others faces(and his WPM is convincingly below Marshall and McGrath who faced a lot of competition as bowlers), so there is a good gap between his and Lillee’s wicket taking ability(considering Lillee up till 70 tests, was resting at nearly at 5.5 WPM). But this is a very close thing, so I understand why’d you take Ambrose over Lillee in the top tier.Anyways Lillee's huge percentage of home tests and very little in WI (not including WSC) and SC disqualify him from the top top tier. WPM is irrelevant since he wasn't tested enough for those stats to mean as much as other elite bowlers.
However you rate Ambrose as an away bowler, he simply was better tested and his other shortcomings in his record mean less against Lillee who has a huge hole.