Honestly it's supposed to make it so you can use your strongest form in base and with more power to boot. It's why in Gohan's case he had to power up as if going into an SSJ form, but not actually transform.
So if current Goku got this power it would likely at minimum allow him to have SSJ3 power in base if it only works with mortal forms. Then he would likely still be able to go SSJ God making a SSG but as powerful as SSJ3 Blue.
Based on that fact he'd at minimum be mystic SSJ God. Which would be SSG x 400 but with all SSG abilities and speed. Then given the fact that evolving the mystic form allows one to break their limits, they'd unlock a new form. Again if it's only mortal forms that get considered then, for Vegeta and Goku being true saiyans, it would likely further their saiyan forms and unlock SSJ4. Then by using SSG in conjunction with that, they'd be SSJ4 Blue. That's as far as they'd go, that is if we only allow mortal forms to be directly enhanced.
If we consider that mystic might enhance any form that is the highest, even if it's divine forms, then we'd get something truly amazing. Mystic in Goku's case would allow him to use TUI in base form. He'd then be able to stack SSJ forms with TUI given the TUI power is in base. Vegeta would then be UE in base and stack SSJ forms with it.
And then by evolving their mystic power, they'd perhaps do one of two things. Either they'd evolve their saiyan trait again, and get SSJ4 to stack with their TUI and UE base forms, or they'd unlock a new evolution of TUI and UE that is unique only to saiyans with their full potential unlocked.
TLDR: Either you get SSJ4 Blue, SSJ Form of TUI and UE possibly a SSJ4 variant, or you get a saiyan unique TUI and UE evolution that only full blood mystic saiyans can get.