How do you keep the main teacher from getting out of sync with the students in the breakout sections? If a student needs 10 extra minutes with their group teacher to understand what is being taught how do they get caught back up to what the main teacher is covering? The only way I can see this working is if the main lecture is prerecorded, but at that point you just have a normal class except for where the group teacher has to do extra work to understand how the main teacher is explaining things. This may work better, but I'm not sure if it would be a massive improvement.
Furthermore, unless we suddenly hire a lot more teachers each student's group is going to essentially be their normal class and we are back to the whole class being kept back when someone needs extra help. Again, the only difference seems to be the main lecture is performed by someone besides the students' teacher. This could be better if the new teacher really is "world class", but I doubt it will be an extreme improvement.