Bike porn

We are not comparing the bikes on the same route. We are comparing the bikes on their respective territory. We'll assume the elevation difference is the same so potential energy is equal. (rider+bike weight is close)

A MTB going down the A-line at Whistler (considered a fast track right?) will average 30 kph if the rider is pro. A roady going down Tourmalet will average 60 kph if he is pro. Difference is, the roady will probably have to avg well beyond 200W to do that. Something the MTB don't because of the technical riding.

The roady however will go 80-100 kph for almost a third of the time. MTB probably wont exceed 60 much of the time. So air resistance become much much greater on the roady since air resistance is exponential in third (assuming no wind). A MTB is far less aero though, but not enough to stir things up.

Rolling resistance is probably 3-4 times greater on the MTB so I don't think it'll exceed 100W on a typical run.

All in all I can't quite work our where the power of the road rider puts the equation. Might be closer that you think.

Another difference will be the periodic heavy braking the road bike has to endure. For a disc system that would mean large temperature fluctuations compared to MTB. Can't be an advantage.

I believe you are thinking this the wrong way. Just to put an example, Runner X runs at 30km/h for 500 meters, then rests for 5 minutes, then runs again at 30km/h for 500 meters. Now compare with Runner Y, he runs 1000 meters at a constant speed, taking the same time than the Runner X. Who would be more tired?
Same thing goes for brakes. Intensity matters, is not just an energy exchange, specially if you are working near the limit.