I can't really see a problem cutting in if someone if doing a tour of the county on downwind, as long as it doesn't cause them to go around. Thats plain rude. The trouble is deciding what they are going to do, especially if they are non radio. I'd be inclined to follow in this case.
I had a similar thing at Gamston in a TB20, a 152 or something decided to extend miles downwind, which caused us problems. As the TB20 is faster than the 152, we had to extend even further so we didn't run up their backside on final. Before you know it you're low, slow and miles from a runway
