What most don't see is that if it is a Carbing, Mazdaspeed, or RS-Aisawa 3-point it actually DOES mount to the firewall, the plate in the center gets attached at that point. But you cannot see it from most photo angles straight on.
this is the most common angle photo, however it "looks" like it just mounts to the thin sheet metal in the cowl area.
let's take a closer look!
take a closer look on my old Miata, here is a side shoot, but you still cannot see the center mounting point
here it is up close, can you see what it attaches too on the backside?
and one from above, you can now see the 3 large bolts holding it in place and and the 1/4" thick steel plate it is mounted to.
sorry for the crap photos, I took these from my old cardomain page