i wouldnt say worse than gc, but I think you are right.
I think the interesting angle is what would have become of PS3/ X360. Who would have gained more from weaker Wii sales? I would bet Sony but I think it is an interesting thing to ponder.
Rather than compared to the Gamecube's actual sales I mean more in terms of how far they'd be behind the other two, Gamecube wasn't that far behind Xbox but I think a hypothetical Nintendo HD 7th gen console would've been a long way behind both the 360 and the PS3.