I would not worry too much. Rip off the run flats as soon as you get the car and store them until you come to sell the car. Put non run flats onfor best performance and better tire selection. Being JCW you really should consider dedicated performance summer only tires - Continental Contact are excellent and run smoothly. Of course that means dedicated performance winter tires too but if you go JCW it makes sense to have an optimized connection to the road.
Rims are more important. If you plan to give the car a good run and sporty drive on regular roads then get 17" rims and you will replace bent rims less often. If you plan to use it on a race track on weekends and drive carefully when on regular roads then the 18" larger rims make sense.