I've seen a few fiberglass soled shoes with SPD mounts, but all the super stiff carbon ones I've come across seem to be 3 hole only. Otherwise I'm pretty happy with SPD. If my new shoes end up with both hole types, I'll be keeping SPD for a while. My shoe choice will override any pedal system choice right now.
It's not difficult to find expensive carbon-soled mountain bike shoes.
1. Go to any bike shoe manufacturers website - let's take Specialized.com:
http://www.specialized.com/ec/en/bc/...019&pscid=1009
2. Go through their menus to mountain bike shoes -
http://www.specialized.com/ec/en/bc/...1009&scid=1019
3. If you find the most expensive one, it's always carbon -
http://www.specialized.com/ec/en/bc/...jsp?spid=56864
4. Usually the midrange model has a carbon sole as well -
http://www.specialized.com/ec/en/bc/...jsp?spid=56688
You could be right that the "super super stiff' carbon models only in in 3 hole road shoes. But there's certainly plenty of stiff mountain shoes out there.