The effeciency depends a lot (if not everything) on the quality of your assistant. If he has judging ability and potential stats of 6 or something, then I would do it yourself. I now have the scandinavian scouting guy (forgot the name) as my *** manager, he has 18 for both stats and I let him (succesfully) manage my youth and reserve contracts. But you have to take into account, from the moment your assistant thinks the player can be just a decent player or better, he'll offer the player a contract most of the time. So if you only want to keep the real good youngsters then I would do it yourself.