I think speedy gets hesitation quite fast when leaned out. when I was using Harri's eco map, I had to richen couple of cruise areas up, because it felt little hesitant to me.
even before misfiring the good mixture will burn more efficiently. lean mixture is harder to get ignited, so you may end up with bigger throttle input to get the same power to reach that 60mph.
lean mixture would also need higher ignition advance, because it doesn't start burning so easily or as fast?
I had one BMW car and I made the mixture richer by adjusting the AIR meter. it made it more powerfull, and also got better fuel efficiency.