355 rwhp SAE is about as good as any M3 has done on a dynojet. It does look like BMW has been fine tuning the software for more consistent and better performance.
SAE is very roughly about 2-3% less than STD. They are methods of standardizing uncorrected, real world, actual that day results to a comparable power level by adding or reducing power to what it would be under certain air density (temperature, humidity, etc) conditions. On a cold day, a car will actually make more power -- what you feel in winter is real. And on a hot day, the car will actually make less power -- and you feel that also. Rated engine power is just output under standardized conditions.