Does the mini cooper 2007 have to use premium gas? What would happen if I used regular gas instead?

Yes it calls for premium. With higher compression engines like yours higher octane fuel is needed to keep the fuel from predetinating before the piston gets all the way to the top of its stroke. Use of lower octane fuel will cause the fuel to ignite prior to the piston getting to the top of its stroke. Fuel economy and performance will suffer if high octane fuel is not used.

http://auto.howstuffworks.com/fuel-efficiency/fuel- consumption/question90.htm --- check this out

What was said above is spot on. If you have a Cooper S then you have to run premium fuel. If a non S then 89 is the lowest you could use.