The first time Goku & Vegeta fused with the Potara, it resulted in a truly OP fighter if we include anime filler. Base Vegetto was stronger than Boohan who himself was leaps and bounds more powerful than SSJ3 Goku & SSJ2 Vegeta, the duo being unable to defeat ordinary Super Boo let alone Boohan. If we apply some basic numbers to them then it might look something like this.
SSJ2 Goku/Vegeta: 10
SSJ3 Goku: 40
Super Boo: 120
Boohan: 320
Base Vegetto: 400
Vegetto is around 4,000 times stronger than Goku & Vegeta individually by these numbers.
As for the second time, Vegetto's strength isn't quite so absurd. Rather than fight in any form before it, Vegetto went right for SSJB, implying that it was required, and had a noticeable advantage over Merged Zamasu. Additionally, Goku by himself managed to break from Merged Zamasu's clutches with a Kaio-ken x20 so we have a slight idea as to where Goku & Vegeta as individuals stand. The numbers might look like this.
SSJB Goku/Vegeta: 10
Merged Zamasu: 200
Vegetto Blue: 300
According to these figures, Vegetto is around 30 times stronger than Goku & Vegeta as individuals which is a significant drop compared to the last time they fused.
Does God Ki affect the Potara fusion in anyway or have Goku & Vegeta simply brought out enough of their dormant power themselves without the Potara needing to do it?
