The MINI brochure that was made available at the Chicago Show discusses the top speed. Under the S header it says the 135 MPH top speed is controlled by a chip whose location shall remain secret. Hard to get to? and is a faster 'upgrade' available?
It is apparently programmed into the ECU to prevent you from blowing the engine by revving it too high. Remember, there is no direct throttle connection. It is drive by wire, so all engine functions are controlled by the computer. Superchips and others have some chip upgrades, but you would have to ask them if they increase the top speed or just shorten the time it takes to get there.