Id be inclined to agree, run lleyns here as well but have never crossed them on the cheviot x blackie ewe. I think the lamb would lack size and wouldn't sell well in the store ring but I could be wrong. Did cross the lleyn on the blackface ewes for a few years but the ewes were very small compared to the cheviot blackface ewes and they were brutal for 3's and 4's weren't uncommon. Great mothers though.Stick with what you've got.
Adding Lleyn to your mix with give you multiples....been there, done that. soddin 3s and 4s.
.....and I'm guessing thats not your kind of ground or farming.
I keep both - (old fashioned) blackies and (hill northish) cheviots.
Happily, they're on different ground - sbf on open hill, chevs see more inbye/enclosed.
The cross is an outstanding sheep, I can breed my own 'crossing' tups, and it's a way of weeding out any trait I don't like in sbf
I see your problem, wanting to run one group.
We've dabbled in mule breeding, but the bfl was costing far too much. but it has made me consider ditching the bulk of the pure chev flock, and replace them with crosses from sbf drafts.
Watching with interest....and want to see pics of these wicklows!