Apparently Mega Instinct Vegeta and UI Goku were on the same level at first and didn't turn out to be any difficulty for Gas.
Then the combo hits started to make it a bit difficult for Gas. Everything seems to indicate that Mega Instinct Vegeta had become stronger.
Afterwards, Mega Instinct Vegeta charges up a Hakai which manages to give Gas a hard time. Then with the help of UI Goku, he just gives Gas a little more trouble, until Gas fights with all his power and throws the Hakai back at them.
So far, everything seems to indicate:
Gas (FP) > Mega Instinct Vegeta (Hakai) + UI Goku > Mega Instinct Vegeta (Hakai) >>> Mega Instinct Vegeta (Power Up) >>> Mega Instinct Vegeta (Initial) = UI Goku.
Finally we see that at the end of the Chapter, Gas was very easily defeating Mega Instinct Vegeta, until the latter increased his power again, to the level that Gas was impressed that Mega Instinct Vegeta was getting much stronger in combat .
Gas (FP) > Mega Instinct Vegeta (Hakai) + UI Goku > Mega Instinct Vegeta (Peak / Chapter 84) = Mega Instinct Vegeta (Hakai) >>> Mega Instinct Vegeta (Power Up) >>> Mega Instinct Vegeta (Initial) = UI Goku.
My question is if UI Goku got stronger the moment he sent the Hakai to the sky?