What if Old Kai....

Used his unlock ability on current Goku and Vegeta? How powerful would they become?

I'm not sure if he could. They don't have like, hidden power that comes out. Gohan it made sense with because he had all the power that he could only access with anger and once he got older it was harder and harder to get that angry easy.

In the anime, Old Kai was going to use his unlock ability on Goku, who seemed excited at the prospect... But he didn't have enough time to do so before the ToP started.

...So I'm sure Goku/Vegeta would gain *something* from the unlock. How much they'd gain is entirely up for debate, though.