Yeah, I was thinking of Gomasu as well, but went with Zamaku as to avoid confusion. As for your question, let me start off by saying I'm not an expert on Dragon Ball, merely a fan. That being said, my personal opinion is that after watching the latest episodes of Dragon Ball Super (56 and 57), a Potara fusion between Black and Zamasu would make them vastly stronger than bot Beerus and Whis. At the very least such a fusion should provide a power boost sufficent for him to surpass Beerus and at least rival Whis, whose power has thus far only been hinted at. For a long explanation, read on.





In justifying my opinion, I'd like to briefly discuss how the Super Saiyan Blue (SSJB) forms of Goku and Vegeta stack up against Beerus and Whis, particularly in the Future Trunks arc. In an often quoted interview between Kanzenshuu and Akira Toriyama for the movie Battle of Gods, he stated that on a scale of 1 to 10, if Beerus where a 10 then the red-haired form of Super Saiyan God would be a 6 and Whis would be a 15. Now assuming this scale is linear and that SSJB is stronger than Super Saiyan God, then it would have to fit somewhere in the gap the between 6 and 10. This seems to also fit Whis assertion at the end of Battle of Gods that Beerus used 70% of his full power against Goku, though the anime version doesn't give an exact figure.





With the introduction of Super Saiyan Blue during the Resurrection of F arc, and Goku's SSJB 10x kaio-ken in the Tournament arc, many fans have been scratching their heads as to how Goku and Vegeta have failed to yet surpass Beerus. Toriyama has stated in the past that he has no plans for either Saiyan to surpass Beerus and Whis in the near future. That being said, if by the Future Trunks arc of Super, intensive training with Whis and three years in the Hyperbolic Time chamber ( Room of Spirit and Time) haven't allowed Goku and Vegeta to surpass Beerus yet, they must have at least closed the gap. For the sake of argument, let's say that by Episode 56-57 that Goku and Vegeta have reached an 8 or 9 on Toriyama's scale.





Judging by how the fights in episode 56 and 57 played out, I would say that Super Saiyan Rose Black is at least on par with SSJ Blue, if not a bit stronger, as he clearly dominated his fight with Goku. Zamasu on the other-hand seemed to be given a bit more trouble during his fights with SSJB Goku and a significantly weaker Trunks. Let's say he's on par with or a bit weaker than Goku.





Here's where a bit of math comes in. If a SSJ Blue Goku is equal to 1, we can conservatively say that SSJ Rose is at least equal to 1, as is Zamasu more than likely. Now, from what I recall, Potara fusion differs from the Fusion Dance, in that the latter multiplies the power of the lower power level by a set value, whereas Potara fusion first adds the power levels of its "fusees" before multiplying it. This resulting in a significantly larger power boost in most cases.





With a little research I've found unofficial multipliers for Potara fusion, based on uncertain or half-remembered scans of promotional material. If these multipliers are accurate (which is debatable) then the most conservative of these estimates place the power boost as (A+B)x8. So if we say that Black is equal to at least 1 as is Zamasu, then the resulting Potara fusion [(1+1)x8] would result in a being possessing roughly 16 times the strength of a full-power SSJ Blue Goku at the moment. So what does this mean for Beerus and Whis?





Here are some assumptions. 1) Toriyama's Battle of Gods scale is accurate. 2) that a full-powered SSJB is about 80 to 90% of Beerus at full strength. 3) Beerus hasn't gotten significantly stronger over time. If we take those assumptions as being accurate, then our friend Beerus would be roughly 1.1 to 1.2 times stronger than SSJB Goku and SSJ Rose Black. Whis on the other-hand would be 1.6 to 1.8 times stronger.





After his Potara Fusion, Zamaku Or Gomasu if you will) would be 16 times stronger than Goku, 13-14 times stronger than Beerus and 8-10 times stronger than Whis. To put that into perspective, when looking at Zamaku vs Beerus, a gap of that magnitude would be comparable to Namek saga Goku just out of the regeneration tank (~3,000,000) vs final form Frieza at 50% (~60,000,000). It would be utterly one sided.





Now, keep in mind fusion multipliers, Super Saiyan multipliers and power levels are alot of the time either gross approximations or mere guesses. They give you an idea of how characters or forms stack up against each other, but should be taken with a grain of salt. The power levels I gave above were sourced from Daizenshuu 7 and as such are sort of official estimates, though not necessarily canon to the manga. Some Potara Fusion multipliers put an estimate of (A+B)x10 which would make the gap even larger.





Long, rambling explanation, short, if my math is right, and my guesses are a bit accurate then a Black-Zamasu Potara Fusion would have him so vastly ahead of everyone else, including Beerus and Whis, that there would be very little they could do to stop or defend against him. Even SSJ Blue Goku using a Kaio-ken x10 which multiplied his full power by ten times (and not just his base) wouldn't in itself not be enough. Black-Zamasu would still be roughly 40% stronger. This would be more or less equal to the Toriyama's stated gap between Beerus and Super Saiyan God Goku from Battle of Gods.



Hope this response has been at the very least interesting. Thanks for the comment, much appreciated.