Lots of people do this, and report on the feed forums that their birds are "fine". End result, typically, is an "ok" bird - because they never see what the bird could have been, they have no standard against which to judge.
i feed all my mutts a high protein diet their first 8-9 weeks of life before dropping them to a 18-20% protein mix, its enough time for the birds with real potential to be exceptional to start to show it (also, I eat excess Roos, so feeding them like you would raising a CX helps with early weight gain for my table.)
You don't need to follow my example, its frankly excessive for most owners, and I don't recommend it as a general course - different needs, different goals, different feed programs. No one, right answer. LOTS of wrong ones.
Very best to you and your flock.