Vegeta had a good bit in the movie where he was just whining about how his power stayed the same, when Goku's was said to have been improved.
By the rules of the fusion dance, the two who fuse have to have the same level of strength, so Goku would've had to lower his power to match Vegeta's power, who already said to be the same as his Boo Arc self.
So that would make Movie 8 Gogeta = Boo Arc Gogeta(hypothetical)