My best guess would be that a SM would top out at somewhere around 90 or so, obviously depending on gearing and the track, and that even if the riders do get topped out, they aren't going those speeds very long and therefore the drag of a visor, etc. doesn't effect them very much.
As far as protection, I don't think the MX lids are lacking anything. And trust me, I've tested a few.
If you look, they both have similar eyeports, the only thing is the chin bar sticks out a bit further on dirt lids. There's been some discussion on that topic, as the chin bars for a period of time most manufacturers seemed to be getting longer and more jutted, which it was theorized could catch on something as the rider is crashing. I remember the first helmet Fox (a Pilot IIRC) was horrible, it didn't even offer good protection from roost. Arai deals with this with a very smooth and close up chin bar design, go take a gander at one.
Most manufacterers have drawn the chin bar back to reasonable proportions again, and, going one step further, Arai came out with a hybrid a couple years ago that is basically a street helmet with a visor. It's pretty slick.