To answer your question literally: I don't think Sonny was overrated or underrrated in either the film or the novel because his role in the family was subordinate to his father until Vito was disabled. What you saw was what you got. Sonny certainly had personal force and was (according to the novel) a "genius" at "urban guerilla warfare." While he had tactical value during wars, he lacked strategic focus and reflective temperament. I think the novel and film portrayed him as he was, so that's why I believe he was neither under- nor over-rated.

When, finally, Sonny was Acting Don, the results of his Donship were indeterminate--he didn't end the Great War of 1946 either by diplomacy or force. It's not clear that either Vito or Michael would have been any more successful. As Clemenza said, "Dese t'ings gotta happen every ten years or so...bad blood [etc.]."