I did spend 15 minutes on Saturday morning trying on a wheel and tyre for clearance.
Before. Stock MX5 14" wheel:
After. New 15x9 0et alloy trial fitted with a used 225/50 to check amount of tyre stretch and clearance on steering lock, which is all good at stock ride height. The finished car will be lowered 60mm so may require the front inner arch panel joint seams dressing back a little for clearance:
Not a huge difference in rolling radius and only about an inch in height:
Bonnet finally collected from eBay seller (he had £20k in his unfinished Mk1 Escort...), it will need to be combined with the rear edge of an MX bonnet to lengthen: