I purchased 10 tupped Lleyn lambs for lambing this year from mid april but actually they lambed first 2 weeks May, which was well after my own flock had lambed. On this experience I would do it again. I did purchase scanned singles and only fed hard feed about 1 week before I brought them in (more to train them to the bucket - they had obviously NEVER been fed). We experienced 1 barren ewe (lost after scanning), 1 died / head first (probably my fault, if I had got there earlier I may have saved it) and another head first but saved it as I assisted earlier. Output 8 / 10 with 7/10 easy or non assisted lambings.
The breeder was experienced and I trusted his judgement that the ewes were well grown enough to do this ie 2/3rd mature weight. However, as I have 2 flocks (shetlands and Lleyns) I will use the shetland tup as much for an easy life / lambing on any ewe lambs going forward. All my other ewes were gimmers/ first timers and I didn't notice any difference in mothering abilities between them and the hoggs (if anything would put the Lleyns of either age slightly ahead). Its very difficult to look at a lamb which is as big or 90% of dam size and not think they need to be tupped!
PS I'm a novice - 2 ewes last year and 22 this year before the hoggs lambed. The key to me is pick EASY LAMBING BREEDS with EXCELLENT MOTHERING ATTRIBUTES! Also help the ewe by penning it for a few days and making sure there is good access to the teats for the lamb. I kept them as a seperate flock as they lambed later which allowed me to give them a bit extra feed even after turnout. I never gave any creep feed to the lambs.
I can't comment on growth of the 'tupped ewe lambs' v the other untupped ewe lambs I purchased - but it will be an interesting comparison if I get around to it! However I am quite happy to have a 'compact' ewe.