No - though you are at a level they likely don't benefit from it, meaning excess protein is likely being deposited as urates and excess nitrogen (ammonia),
But "too high", no, not as in dangerous, merely "too high" as in probably more than they require for optimum health.
I raise mine on 24%, they are dual purpose mutts, but not banties. If I routinely raised meat birds, I'd look for other options. I probably should for my turkeys, but only have 4 and I'm not keeping them long - just until they are sized to make a good meal for the wife and I.
and I will say that you should take all of the above with a grain of salt. There are essentially no studies on bantams, and I have no experience with them personally. The above is either the results of study with production birds, production meat birds, or the collective experience of a lot of us raising dual purpose mutts in dissimilar conditions. The biological processes are the same - but a bantam's actual needs? I'm assuming they are similar to other dual purpose chickens with substantial feather/mass ratio, and that they eat less per day. But that's assumptions on my part.