Well, actually we have, and do, run pure GOS, pure saddlebacks, and OSBs. We had a saddleback back from the butcher last week, and have two OSBs going of the week after next. The last saddleback came back at 70 kilos dead weight (pork) and 10 kilos of sausages. It was killed at 8 months old, and was a very good quality carcass. It didn't blow our budget. We also AI the ped saddleback sows to a pie train boar, as, just like you say, it gives a great carcass conformation, and with it being half saddleback, and outdoor reared on a free range system, with a good wholesome diet the taste is excellent. I would imagine many would struggle to tell you it wasn't a traditional breed, it certainly doesn't taste like it came from a super market. What people DO notice, is that the chops are larger, the joints fuller etc.
The other advantages of the x bred pigs we keep, is that you can run them on as long as you like and has a lower risk of running to fat.
P.S Do you really feed your sows 20kg of feed a day? And not mate them until 2 years old. We have one with a litter of 11, her second litter, by age 2, who is certainly not fed 20 kg a day, and was pronounced by the vet to be in excellent condition.
We run a small scale set up, but everything here has to be justifiable financially.