I have a batch of Red Ranger/Dorking mixes outside and the biggest roo weighs 8 lbs on the hoof at 15 weeks.
We have some Red Dorking and butchered the smaller male at 22 weeks (Christmas time), and it was not quite that big - meaty and tasty, but not that big. The BJG was quite big, but thats what they do - grow big and mature slowly. So its great that your D/RR mix are doing so well for meat within a relatively short period of time!