I drive a
MCS and a Honda Insight hybrid. The MINI would never make a good platform for a hybrid because of it's weight/drag characteristics. The Insight has a .25 Cd while the MINI is around .36. The Insight weighs 1800-2000 lbs while the MINI is over 2500. The diesel version is your best bet for economy (if not emisions).
You just can't make a normal car into a hybrid. It must be designed from the beginning as one. You may get better acceleration from a smaller ICE but you will never get the economy. Also, there is a learning curve for driving a hybrid. They cannot be driven the same as a regular car. You have to be very "in touch" with your car like a professional racecar driver but in a different way. Many people don't have the time/patience to learn how to drive for maximum efficiency. I've been able to get great economy in my "S" by using the same techniques as I use in the Insight - things that are just not noticable in a regular car.