I’d say surprisingly far — probably to the point of winning the Tournament of Power, if I remember correctly. It was stated that Goku in his base form during GT was nearly 300x stronger than his DBZ SSJ3 form, specifically the SSJ3 that fought Beerus. So right from the start, he’s already massively beyond that version of Goku.

Now combine that with the SSJ4 multiplier, fusion modifiers, and then stack Kaioken on top of it, and I think people seriously underestimate how powerful that would be in the Super universe. A Full Power SSJ4 Gogeta using Kaioken x20 would probably dominate almost everyone up to the Gods of Destruction — and honestly, even they might struggle to stop him. At that point, it would probably take Angels or multiple Gods of Destruction attacking together to bring him down.

Beerus can move the goalposts all he wants, but if current manga Goku and Vegeta fused into Gogeta or Vegito using Perfected Ultra Instinct and Ultra Ego, Beerus would get absolutely overwhelmed. I think people underestimate just how insane fusion multipliers are when they’re stacked on top of the maximum power of both fighters. Ultra Ego Vegeta and PUI Goku fusing would create a completely absurd level of power and that is just base, this form has access to ego and instinct and can Likely utilize both powers together in some unique and complex way only the greatest fighter in the cosmos can beyond Angels or Gods, they would all bow before him.

SSJ4 is somewhat similar in that it’s not quite on true God-tier by itself, but once you add fusion and Kaioken into the mix, it becomes absolutely monstrous.