I am not going to state any actual multipliers, but I think I can try to create a list of different kinds of fusions and why one form of fusion is stronger than the other. I'm going to take into consideration, the Namekian fusion, The fusions when Cell/Buu absorb another character, the fusion dance and obviously the potara fusion.
In terms of power, I believe the correct order would be
- The Potara fusion: The potara earrings are worn by the Supreme Kais, the supreme ruler of the universe and its something which is not possessed by any mortal. It was stated by the Elder Kai himself that the Potara fusion was stronger than the fusion dance. As for the kind of fusion where a character is absorbed, the potara fusion is still superior to the same. This can be explained by going back to the Buu Saga in Dragon Ball Z. Ultimate Gohan was the strongest unfused character than and a lot stronger than Super Buu. Super Buu was a lot stronger than SSJ3 Goku. The SSJ3 transformation is undoubtadly a lot superior to the SSJ2 transformation and Vegeta in his SSJ2 state(Without majin) was slightly weaker than Goku. With the potara fusion, Base Vegito was able to fight Super Buu with ultimate Gohan like it was nothing. Even the recent Dragon Ball Super episodes where Kefla reached new heights managing to rival the power of SSJB + Kaioken Goku and possibly even surpassing it, indicate how powerful the potara fusion is.
- The Fusion Dance: It has already been confirmed that the Potara fusion is superior to the fusion dance, however, I believe the fusion dance is superior to the kind of fusion where a character is absorbed. Since Gogeta is not considered canon, I'm only going to use Gotenks to prove my point here. Firstly, we know for a fact that Base Goten and Trunks were no where as strong as Base Goku, Vegeta or yet alone Gohan(Who was rusty at that point in time). Fat Majin Buu was considered to be one of the strongest characters at that point in time and the only character who was capable of defeating it was SSJ3 Goku which is a 400 times base multiplier. Once Evil Buu absorbed the fat majin Buu, he was a lot superior to SSJ3 Goku. We've seen android 18 being able to fight trunks and goten together while they were using SSJ during the tournament. Even after training in the HTC, it's obvious that base trunks and goten were no where close to base goku, gohan or vegeta in power. However, Base Gotenks was a lot stronger than Base Goku such that SSJ3 Gotenks was equal to super Buu in strength. We haven't seen such a massive power increase by just absorption. If you go back to the cell saga, Cell absorbed Android 17 and 18 and let's say the two of them were as strong as Super Namekian Picollo(After he fused with Kami). However, Goku in his Mastered SSJ state, just a (50 times multiplier) was able to put up a decent fight against cell.
- Absorbtion based fusion / Namekian Fusion: Based on power levels from the earlier seasons,
we know for a fact that the namekian fusion is (Namek A+Namek B)*7.09.
The absorbtion based fusion seems to be of this nature with a very low multiplier, or probably only of an (A+B) nature. My proof for the latter would again be based on the the fight between Buuhan(Super Buu+Ultimate Gohan) and Vegito. Super Buu was a lot stronger than 400 time's Goku's base strength. And Ultimate Gohan was a lot superior to Super Buu's strength. Even if the potara fusion is an incredibly high multiplier,
compared to the absorption based fusion, there is no way possible for Base Vegito's power to be comparable to Buuhan's if there was a higher multiplier incase of the absorption based fusion. We know after Kibito and Shin fused, (Just before Goku went back to earth to assist Gohan fight Super Buu), the old Kai stated that Kibito was still a lot weaker than Buutenks. So the potara fusion multiplier is not astronomically high which would only imply that the absorption based fusion has a low multiplier which is why Buuhan was completely dominated by Vegito. With regard to whether the Absorption based fusion or the Namekian based fusion being stronger, we can't really say with a certainty. This is because, we would be comparing Picollo to the likes of Buu and Cell(Who were technically the strongest characters in their respective arcs, until they were surpassed by the Saiyans). Also the characters who were absorbed were a lot stronger than the characters picollo fused with, so I don't think there is sufficient information to distinguish the two