Why is that? I mean, I'd agree that it certainly isn't controlling/determinative, but it is certainly relevant. If you compare where Sexton is now with where Cassell was at the same point in his career, and Sexton is better, then the only way Cassell is Sexton's ceiling is if Sexton is certain to improve less than Cassell did. That just seems off to me. "Ceiling" implies best case scenario, and that's not it.
All that being said, I think comparing offensive numbers alone doesn't tell the full story. Cassell was a significantly better defender in his first couple of years in the league than was Sexton, so Sexton would have to improve substantially on the defensive end. It's always possible that he'll do that, in which case he may well end up better than Cassell.
But I think talking about ceiling/potential isn't all that useful absent also talking about how likely someone is to reach it.