I've noticed that a lot, cockerels of the same breed hatched the same day can be greatly different in maturity. You can often get a pretty good idea of testicle size from their body size and how they are acting. That's a big part of why I say that even the same breed can have greatly different maturity at the same age.When butchered, the testicles of the Dorking were quite a bit larger than the BJG, also indicating that the BJG was not yet mature.
Like everyone else I'll be interested in your comparison. Not just taste but how much meat you actually get. I'd expect the JG to be quite bony, but that doesn't necessarily men less overall meat. I'l be glad to see the comparison.