I know this is an old argument, but I'm going to start it again. After running back to back comparisons this weekend and today, there is no way to equate the performance of a Road Bike with a mountain bike.
The road bike is at least 30% more effecient than the MTB on the same paved terrain, possibly even more than that.
Your going to have a hard time telling me that 6 lbs difference in bike weight for a 220lb rider explains any of that.
On steep hills, once the legs get tired the two types of bikes begin to equal out but on lesser hills there is a tendency to fly to the top without loosing speed. On flats, the road bike wheels just coast and coast without much loss of energy.
I guess it would be interesting to try a set of 23mm tires on 700c wheels on an MTB frame to get a feel for how much of the road advantage is contained in the wheel tire combo.