The more likely shift will be from current 3.7 Roadster owners.
This group are already in the £50/£60k zone, and want that performance level.
Current trad Plus 4 owners were either not looking for Roadster performance, or not wanting to spend Roadster money. So I'm not convinced that many trad Plus 4 owners will make that change.
Interesting point Dave. When I ordered my +4 4seater I drove the Roadster 4 seater at the factory. The extra cost was £7 K and, though I liked the performance, thought the lighter Duratec with alloys would handle better.
The difference in price between a newish Roadster and the new +4 would make me think.