one thing to consider about premium vs. regular is the configuration of the engine. Losing performance may be a certainty, even for engines with the proper knock sensors and dynamic engine management systems. You may LOST horsepower, but it will the engine will adjust and keep knocks to a low. But, if there is NOT a knock sensor and proper engine management system, not only will one get knocks but also less than optimal engine running. Performance will most likely be a reduction when going down to regular anyways.
So, my thinking is, the smart and other cars ask for premium, because they know they don't have the more advance EM to adjust for lower octane fuels.