Since 17 and 18 would have perfect compatibility according to Beers, how strong would their Fusion be, compared to other ToP characters? Would they be stronger than Fusion Zamasu?
It is true though, the Potara boost seems to be completely random (at least for Toei anyway) as Kefla is considerably stronger than SSB Vegetto despite her components being below SSJG.
If No.17 is able to hold his own against Blue tier opponents that puts him pretty high up there for power. No.18 never showed she was above the base Saiyans by very much. If they receive the same boost as Kefla then No.35 would be KKx10 Blue tier at the bare minimum. I'd say no stronger than the Genki Dama for the maximum.
Stronger than base Kefla, but likely not leagues above SSB tier, if even above it. Logically, they shouldn't be leagues above initial Merged Zamasu who still got overpowered by SSB Galu after a resolve boost, unless the compatability boost is huge.
The comparability boost is also random, Black and Zamasu being the same person should have had the greatest compatabilty boost imaginable yet Kefla >>> MZ even after his mutation.
That's not to mention that both Kefla's components are weaker than MZ's.
Android 17 alone is already SSB-tier; adding #18's strength on top of that while factoring in a multiplier of some form would at the very least make this hypothetical fusion comparable to SSB Goku Kaioken x10-20. Given what Vados had to say about multipliers, I'd say the androids would at the very least benefit from a 10-20x multiplier.
Personally, I feel they have enough compatibility with one another to reach a level that enables them to contend with Destroyer Toppo / Suppressed Jiren.
As for individual power, I feel the following sums it up pretty well.
SSB Goku / SSB Vegeta (current) > / = Android 17 > Ultimate Gohan (current) >/ = SSB Goku / SSB Vegeta (Early Zamasu) > SSB Goku / SSB Vegeta (Universe 6 Tournament) > Golden Frieza (RoF) > SSB Goku / SSB Vegeta (RoF)
Ritual Form Goku >/ = Mastered LSSJ Kale > SSJ3 Goku > SSJ2 Goku > SSJ Goku > SSJ Caulifla > / = Base Goku = Android 18 > Base Caulifa >> SSJ Vegetto (Buu saga) >> Buuhan > Buutenks > Ultimate Gohan (Buu saga) > SSJ3 Gotenks > Super Buu > Mr. Buu