The M+S marking doesn't make them a winter tyre, it just means the tread pattern is suitable for Mud & Snow and not freezing winter weather, for these conditions you need a tyre displaying the "Snowflake-on-the-Mountain" symbol.
M+S marking is officially defined in an EU directive as meaning: “a tire with a tread pattern and structure designed to better performance in snow than that of a normal tire”. The performance of “M+S” marked tires is therefore neither defined nor measured.
The snow performance of winter tires is specified in North America with the so-called "Snowflake-on-the-Mountain" symbol. Only tires which meet or exceed these requirements may carry the "Snowflake-on-the-Mountain" symbol.