Talking to Derek (GazMan) at Brands yesterday, the track car they are testing is 925Kg WITH driver! They have to add ballast to get up to the regulation 960Kg with driver limit. Assume a driver is around 75Kg and you have around 850Kg for a bare empty stripped out race car.
IIRC there was a bloke on the NZ MX5 forum who reckoned he'd got down to ~700kg with fibreglass panels and no interior/heater/lights/roof etc. Whether it was a genuine 700kg I don't know, but he didn't seem the type to make things up.