Based on the following quote it is clear that characters in Star Wars do not believe that Imperial vessels are that powerful: "It's been totally blown away." "The entire star fleet couldn't destroy the whole planet. It'd take a thousand ships with more fire power than I've...". - Han Solo, EPIV "A New Hope"
The Imperial Starfleet couldn't have reduced Alderaan, a major world sporting a planetary shield, to an especially large meteor field in the span of a few hours (and if it had taken longer, Han would have heard it was happening); that is not in dispute. Not outclassing the Death Star does not inform how Rebel capital ships would perform against the Federation, however.