I completely agree, front brakes will always be more powerful than rear brakes, and the front brake is the more important one. My main comparison was between my rear brake operated with a 6600 lever and the same rear brake operated by a 6700 lever. The power and feel of the new lever is completely different. Whereas I expected the differences to be pretty un-noticeable, the power has dropped considerably with the 6700 levers, especially when braking from the hoods (because I cannot compensate by using more hand strength in that position). I'm using the feel of the front brake as a reference, because my front and rear used to feel similar (i.e., a similar amount of firmness / squishy-ness when applied), even though they had different max braking power - the rear brake now feels much firmer than the front or than the rear used to feel, too firm to have sufficient power I would say. The fact that it is my rear brake that has lost a bit of power does indeed make me less worried about the situation than if it had been my front brake.

Your point about lever length is also a good one. When reading up about this, I learned how it is the ratio of the two distances that matters (cable anchor to lever pivot and finger position to lever pivot). However, I believe it was Sheldon Brown who was saying that the distance from lever pivot to the finger position is pretty constant across most levers, and so can be ignored, which is why I did so above. The new Campy levers are therefore interesting in this regard. I just measured the 6600 and 6700 levers and they both appear to be about 76mm between the lever pivot and the place where my fingers rest, the total lever lengths are slightly different, about 109mm and 106mm, respectively, but this shouldn't matter. This therefore doesn't appear to have been a factor in the re-design of the Shimano levers.