There is nothing else in contact with the road except for your tyres. I will chime in with what many others have already asked which is how much is your life, your passenger or any other third/fourth/etc party worth?
I run Bridgestone RE55 semi-slicks all the time for approx 2400 km (2hr monthly cruises) a year and track days or competition. Granted not everyone will do this but even my EVO will be getting RE001 Adrenalin Potenza tyres as I am not satisfied with my "import compliance" tyres.

Buying a sports car like the MX5 with its great handling prowess to put on sub-par tyres is sacrilegious to me.

There is good constructive advice in this thread.
