M ratings are meant for motorcycles, and are a single use (impact) helmet. If you hit it against something, it's garbage. They also have to have wider sight angles than an SA helmet needs.
For HDPE and autox, it probably won't matter (I've heard some autox organizations joke that anything better than a hockey helmet is fine
), but for time attack or any car with a roll cage I would bet that they would mandate SA helmets.
Even some lapping organizers want SA rated... PCA (Porsche club of America) mandates SA rated for lapping days, but the UCR allows SA or M.