Am not sure how much difference carbon soles has made to my performance, but they feel much better and more comfortable over longer distances (no more numbness/hot spots on anything up to 200k). They must make a difference to power transfer as they feel much more direct than my touring shoes - I can feel my foot and the sole flexing all over the place in those.
I've had a pair of Spesh BG carbon shoes (mtb ones) for a couple of years, got a nice discount in the sales from my LBS. I always end up with mens shoes, and also always take the next size up.
A couple of friends have the DHB ones and seem to get on well with them, no idea about sizing though.