Gokou states Buu was toying with him. So yea...He'd really be able to win at full power. How many people in DBZ have said they could win but weren't able. Vegeta against Freeza, Cell against Gohan, Buu against Vegetto....What they think and what is real are two different things. Buu's over there mocking them so yea he's really trying his hardest to kill Gokou. Then Vegeta letting Cell become Perfect, Recoome against Gokou, Burta and Jeice against Gokou. Because it's said doesn't make it true when one is not even trying. If Buu was serious then maybe I'd believe it but he isn't being serious at all.