If Buff Buu = South Kai + Kid Buu's powers added, then for the gap Super Buu/Kid Buu to be massive, you have to accept that South Kai was a lot stronger than Kid Buu.
The first. There was no need for such a massive gap. The kids could have successfully transformed into Gotenks for the first time in the ROSAT against Super Buu and started out the fight as SSJ.
Piccolo said about Super Buu that he was stronger than before, but he doesn't specify whether that increase happened when he transformed or right before splitting.