Precision Sport Industries just developed a removeable roll bar...
http://www.bmwblog.com/2014/10/05/custom-removable-rollbar-bmw-m4-psi/
http://www.bmwblog.com/2014/10/05/custom-removable-rollbar-bmw-m4-psi/

I guess you're not into racing or tuning cars or any of that stuff.What is the point really? Unless you are actually driving a track car, I don't see why you would need this roll bar. Rent a track car, but I think its weird to have a removable roll bar. Are you really going to put it in sometimes and take it out other times. You can't even fit it in there without taking out all of the seats and stuff.