I dug through the rulebook just for some clarification, and motorcycle helmets (like the RF1400 I have) are ok provided they are Snell M2015/2020 rated, with a couple exceptions. If you go 9.99 or quicker in an open bodied car with a power adder (or open bodied electric) it needs to be an SA rated helmet, not M. Pro mod, top alcohol dragster, top alcohol funny car, top fuel and funny car also all require SA rated helmets instead of M. Everything else, the M designation is ok per the rulebook.

That being said I don't know enough about the differences in helmet ratings to know which would be better if your head contacts a rollbar in an accident, as an example.