Back to the C5-0, yes this would be a great addition. All the benefits have been covered here, but if there was a C5-0 available, I'd rarely, if ever, use the C6-0. Estes could bring back the Farside if a C5-0 was available! When I build 3 stagers now, I try to make the 1st stage 24mm and use the C11-0. This works great for a Centuri T-Bird clone, less so for a Farside clone, which has a fragile booster when built for 24mm.
As long as we're at it, let's have a B8-0! AFAIK the B8 used the same nozzle as the C5.