He's weaker than Base Goten, though for reasons established in a similar thread about Trunks, I'd say he'd still be more or less on Pure Boo's level via scaling. He ought to not be too much weaker than Base Goten, considering Goten still felt compelled to go SSJ after getting an off-guard (
) cut.
As for how the boost works, I'd say it's additive for all but Vegeta. There ought to still be some boost when it's a bit difficult to have SSJ Goten remaining at the same power yet having a failed self-destruction more powerful than Ultimate Gohan. We're also shown later on that those controlled by Baby seem to have greater capabilities than they would normally, such as Bra being able to fly despite being implied in the Super #17 Arc to be unable to do so or Dende and Popo being brought to the level Kibitoshits decided it was best to run from them. The additive boost would only be somewhat relevant in Goten's case though when he was the only one of the three Saiyans Baby fully controlled who wasn't far above Baby in general.
As for Vegeta being the exception, it was indeed due to it being a full assimilation. Baby makes it quite clear his end goal after taking over Goten is to take over Vegeta's body, both due to him being the strongest fighter on Earth at the time and likely for the irony of having the descendent of the one responsible for the Tsufurians' extinction become the base for the race's revival. As we see later on when he exits Vegeta's body, what mutation has occurred is immediately erased and it doesn't seem that he plants an egg like he did with his other hosts, so making sure this body would be that of his end goal would be essential.
