I think its around the same, I have welsh mule ewe lambs, meatlinc cross ewe lambs, charmoise ewe lambs and charolais cross ewe lambs in lamb this year to a charmoise ram
Also I have not found that the productive lives are shorter, you get an extra lamb in the first place and with some ewe lambs I find they do alot better tupped young, some charollais crosses or others get far too fat when run over winter empty, I have some lambing in 6 weeks first time as shearlings and they are far far too over conditioned, should have tupped them last year
hopefully no problems at lambing
They do eventually catch up, my biggest ewes now were tupped as ewe lambs
edit: perhaps its different with things like southdowns