Our very first year we did- Feb born Wilts Horn went to a lleyn and lambed the following April with no apparent problems- all singles which is I understand the best outcome. Since then we haven't done it, largely because the consensus seems to be not to, but also because it felt a bit wrong to put our own produced lambs to a ram when to us they were still babies. However, this year we have given it a go, mainly because for the last few years they seem to get a bit fat if we leave them and so far all our first timers have only ever produced singles, even after waiting a year- which means that we can have a ewe for nigh on 3 years for the measly return of one lamb.
So we have Wilts X Lleyns in with a Wilts ram. I guess the main thing is to not try to breed giants. One thing we have ( or haven't) done is feed them .I have read that trying to artificially inflate their size leads to increased risk of twins and associated problems. the advice we have had is to let them get there in their own time - or not. The ram leaves on Saturday and so far only one of the 3 ewe lambs has been covered, still time but maybe they're just not mature enough