I don't know that feed - what's the composition and protein content like?
Traditional breeds don't need a high level protein, somewhere between 12 and 16% is where I'd be aiming for - higher than that they'll just lay down fat.
What age they are and the quantity they get will also have a lot to do with how they grow. The old rule of 1lb per month of age (up to 6 months) is the easiest one to follow, but with GOS & SB I'd probably reduce it a little. Keeping them free range will help with condition - if they have lots of space to roam they should hopefully be nice and lean. Unfortunately the only way to know for sure is when they're on the butchers hook
but if you can't feel their ribs or backbone it's time to cut back
HTH