Nishant made a good point about universal acceptance of a Pope being the criterion for legitimacy. It make sense as the a priori establishment of the dogmatic fact of legitimacy. I think about this often and haven't come to a final resolution on the subject.
Let's say that a Pius XIII came along and declared BoD to be heretical. You, I suspect, would just reject Pius XIII as a heretic and thus refuse his dogmatic teaching. If this hypothetical Pius XIII came along and delcared BoD to be dogma, then I would immediately reject my former opinion and accept the teaching. Thus the difference between sedevacantism and sededoubtism. With straight sedevacantism established as permissible you couldn't have any a priori guarantee of the truth of any dogmatic papal teaching.
By the way, Sneaky, if you were to convince me that Suprema Haec represents true Catholic teaching, then I would immediately cease to be a Traditonal Catholic and would accept Vatican II as substantially free form error. Every error and heresy that can be attributed the the V2 papal claimants can be traced back to and logically follows from the Suprema Haec ecclesiology.