Personally I wouldn't cut them down unless they are rolling fat.
If you reduce the creep, they need to replace that nutrition from grass, which means bigger bellies yet...

And/or, losing condition. Hence mine get more creep when they are on poor grass, less when they're on better grass.
When I say 'creep', mind, mine are getting a 16%-17% ration suitable for tups and lambs, not a high protein feed designed to push them on. So one other option would be to replace the creep (if it is a high protein, expensive lamb creep) with Champion Tup Mix or similar, which will help them get enough nutrients without blowing up but be cheaper for you.
If you do reduce their hard feed, do keep an eye on their condition score - if they go backwards, I'd give them more cake.