Soy, to frame the same question in a different way:
When you watch Davydenko/ Berdych/ Safin trading hard backhands from the baseline (don't count highly spun ones, or ones where they deliberately cut the pace, dropshots etc), and compare to Gasquet or Federer, does it appear that they are hitting harder ?
I think the 2HBH players are applying more force than a OHBHer, all things being equal. But more force does not necessarily equate as hitting a faster paced ball. The 2HBH is practically a weak hand forehand with the strong hand as the stabilizer. Because of its forehand-like nature, players with this backhand are more apt to hitting aggressive with it at a more frequent rate than those with a OHBH, typically. To hit fast, much of the pace needs to be generated by the player.
The OHBH is good at feeding off of the opponent's pace. As a result, not as much force is needed to return the ball as a 2HBH, but timing and coordination is even more critical for OHBH players.
I think the
average 2HBH shot goes faster than a OHBH shot even if you discard situational shots like drop shots/lobs/slices. The 2HBH has a bigger comfort zone for hitting hard as it has an easier time with high balls. However, the OHBH is struck sooner than a 2HBH which
can result in less reaction time for the opponent. Gasquet's backhand requires him to setup sooner than most of the other OHBH pros, but this allows him to generate sharper angles while giving his opponent less time to react.
In short, a 2HBH requires more force, but not necessarily generate more speed. Average speed would likely go to the 2HBH. Max speed might even go to the 2HBH, but the early preparation time and execution of the OHBH would cause just as many problems as a 2HBH shot. Because the 2HBH stroke is hit slightly later than that of the OHBH, force is more critical for the 2HBH. That's probably the main reason why I would likely lean to saying the 2HBH is hit harder.