Um. I don't really know the answer to you questions about breeding. They are certainly a specialist meat strain so almost certainly for the commercial market, I'm sure, but whether they were developed for indoor or outdoor production, I have no idea. I don't think ou should rule them out simply on that basis. Bewsters rpoduce organic, free range chickens.
They grow quite fast, but not as fast kept the way we do as they would in intensive conditions ie 84 days as opposed to 39. Ours are in a run during the day so have access to grass, soil and cabbages (!). The carcase is quite different to, say, a RIR or Light Sussex. It's much meatier, with good breast meat. The flavour and texture is very good indeed.
We've fattened LS cockerels that were surplus and the Hubbards are way, way ahead.