Just my 2p worth, sound like HandyAndy now......
Why not ditch the Mx5 front uprights and buy a new set of sierra uprights from Rally Designs, about £95, and use the Haynes designed front suspension to run the car on. The suspension layout and geometry was meticulously designed, on Lotus own design software, to give an ideal ride between road and track use and is a major design in the Haynes to give it the excellent ride and handling that it has. Obviously would need discs, £18, and callipers, ???, to go with it but i believe this would be the best solution. We are just about to star putting a car together based on the Rx-8 and will be doing it this way.
All the Best....AndyH