There was an interesting pschology experiment repeated on the BBC about a year ago I guess.
They got a whole bunch of kids and arranged two parties: One they fed them only healthy food but arranged boistrous activities. In the other they were given masses of sugary foods but given calm games to play.
The Parents all agreed that after the party with the sugary foods the kids were totally hyperactive.
Only thing was they switched the parties on the parents. So the parents thought the kids had been hyper after the sugar but really they'd been eating apples and cucumbers but playing boistrous games.
Now that's not definative proof but it does illustrate that parents perceptions are probably more important than the sugar.