Captain Cadaver said:For a more refined version of fusion (ie. one where the fusion can transform at will), yes, as it's supported by pretty much every piece of information such as Base Gogeta logically being stronger than Post-Rosat Base Gotenks and SS3 Goku, Base Vegetto's power in the Super manga, etc. For a less mastered version of the fusion such as Pre-Rosat Gotenks, I'd probably say no when it's hard to imagine SS3 Goten being weaker than Base Gotenks when the former shouldn't be several times weaker than Goku in equal forms.
Synchronicity is a huge factor in fusion.
A certain level of synchronicity is a requirement for the fusion to succeed... and it is my belief that the more in-sync you are beyond that requirement, the better the result.
That being said, it is not hard for me to believe that both boys got slightly stronger but the main source of Gotenks improvement comes from Goten catching up to Trunks, becoming his equal.
This is supported by the fact that Goten enters the RoSaT with greater motivation than Trunks and is portrayed throughout the story as more gifted by default.
Also, Goku states that he could not fuse with anyone in the afterlife... why? Surely he could suppress himself. This indicates to me that there is a limit on the difference in innate power between fusion participants.
In short, I believe distractions (i.e. holding transformations, suppressing yourself) and power difference to be factors involved in Gotenks’ Pre-RoSaT weakness when compared to his Post-RoSaT self.
In defense of the synchronization thing, we're shown that fusion can "succeed" on a certain level, even if the dance is performed incorrectly. We're also shown that performing the dance perfectly is not enough to even initiate fusion. Therefore there are varying levels of success and the dance is not the only factor. So, while they may have perfected the dance itself, other factors should still be able to improve the results.
Also, when Goku first begins teaching Goten and Trunks, that practicing in the base form would be easier as doing so in Super Saiyan is more difficult... consider why.
EDIT:
In regard to the whole "they trained as Gotenks" thing... you're right, there's not much, if anything, to support that being possible... at least in terms of power. Knowledge/ability seems plausible though. Gotenks is created, not summoned.