To say ANY of those cars are even close to a production car is a dream. Maybe they "say" they use the same engine block. We all know every part on those car are completely custom, from, engine internals, to suspension, electronics, all the way down the the width of the chasis. As far as the current Z4 BMW said from the very beginning there would never be an M version. Let the each manufacturer develop what they want. Only so much horspower can be useable around a racetrack anyway. I would be thrilled to see the Z4 GT3 race in the states. I think most BMW race fans would feel the same way.
Lone Star Chapter BMWCCA HPDE Event Chair, Texas Trifecta Event Chair, & BMWCCA driving Instructor