The testing from Snell is very much the same for both SA and M .
The MC lid has an additional test it must pass that simulates a puncture from sharp object like you might hit on contact with a guard rail or metal edge on yours or another vehicle.
The tight fitting fire skirt got manufacturers in trouble when after a crash, the driver would upchuck and the fluid could not escape and went into the lungs.
The flat back that was so aero revolutionary for awhile, was found to contribute to neck trauma when it caused the head motion to accelerate as it turned in some crashes.
It was designed and patented by an SCTA racer.
Snell is not the only good standard and all of them learn as they go as does SCTA.