My twin boys are very very tall and I had the same problem of what to do as they had totally grown out of their seats at 2.8yrs. Would just mention though that they were not heavier than their group-seat capacity, just really, really tall.
I went straight into the high backed booster, as you mention with the car seat belt.
The only problem we had, and it was only for the very first few times we used them, was getting the boys to actually stay in the seatbelt as, obviously, the seat belt is not as rigid as the straps over both shoulders and buckled at the crotch type.
I have to say though, that it was literally only a couple of journeys that I had to have my eyes glued on them and stop/start to fix them back in.
They very quickly learned that we don't get anywhere unless their seat belts are on and now we don't have any problems at all.
Only downside of the booster seats, that I can think of, is that, even though they say they recline, I tend to find the degree to which they do recline is not enough to ensure a comfortable position for the boys if they are sleeping. My boys don't often sleep in the car anymore, but on the occasions that they do, their heads tend to drop forward and mummy has to pull-over to try and tilt their heads back into a more comfortable position.
Hope this is useful.