What would you attribute that to? I can think of three possibilities:
1) Stiffness. Not an issue: Dominators are the same sole as Genius 5, for example.
2) Fore-aft position: Could be, but I've found my mtns more able to get further back, which is the direction I'd expect this sort of problems.
3) Weight. The mtn shoes are going to be heavier. Not completely convinced that matters all that much.
Not arguing, just curious in what you think it might be.
Here's what I can tell you.
My mountain shoes are ten year old Sidi's. They don't have a model name on them, so that's a mystery. I can say, however, that I just tried to flex the soles every which way, and I could not do it with the strength of my hands and arms.
My road shoes are S-Works. As it is with the Sidis', I cannot get them to flex. Both pedal systems, BTW, are able to place my feet into exactly the same spot
I can say, though, that with the Eggbeaters, and the Time Atac's and the Shimano I-Can't-Remembers that preceded the Atacs, there was never a time where I did not feel as if my shoe was perched atop a large ball bearing. With the MTB equipment, no matter how I adjusted the pedals and the cleats, whenever I'd pedal with any genuine force, my feet would never fail to rock either clockwise or counterclockwise around an axis at a ninety degree perpendicular to the pedal spindle (I hope I'm explaining this properly). I would also never fail to feel that "ball bearing" digging into the bottom of my foot. I'd never fail not to feel a click when I'd be pulling up on my feet in order to get more power into the bicycle.
None of this has ever happened to me with a road shoe/pedal set-up.
To be sure, a mountain set-up might well currently exist that will give me the best of both worlds -- the security and lack of extraneous motion of a road set-up, and the ability to stroll through the Neiman-Marcus if my wallet feels a bit heavy, that day. The thing is, I'd rather spend the money at the Neiman-Marcus than buy a succession of MTB pedal-shoe combos until I find the one that does the job.
As for weight, it's not a crucial issue for me. In my book, comfort always trumps avoirdupois.