I don't think this has to do with a change in efficiency, or maximum regen.
They've just changed the throttle mapping, both on the consumption (demanding power) and production (regen) side. With 7.0, a given amount of pedal travel produces less power than it did on 6.2, until you get to full pedal travel, at which power is the same as it was on 6.2. In other words, the power increase for a given unit of pedal travel starts out more gradual, then ramps up faster in the later portion of its travel than on 6.2.
The same is true of regen, except in the opposite direction....initially as you apply less pedal the regen comes in more gradually, then it ramps in stronger than before as the pedal continues to come out.
Why did Tesla do this? Probably to give the driver more control at low throttle inputs, a smoother ride, and to decrease impact forces on the drivetrain (gear lash, etc).
End effect to the driver? The input your foot is used to applying for a given regen situation is now no longer enough to provide the same amount of deceleration. In other words, to slow down at the same rate, your foot now needs to pull back more than it did before.
Because of motor memory, when we tend to apply the same inputs as before, we end up just decelerating at a more gradual rate, requiring more braking.
Moral of the story? Try applying more aggressive foot movements during regen than you did before. (By that I mean your foot should pull back more, not necessarily move quicker or be more jerky). If you do this, you should find the regen force available to be the same as it was before.
I don't really like the new mapping. It feels too mushy to me. I much preferred the tighter mapping of 6.2.
This is what I gather as someone driving the S since December of 2012 and coming up on 60,000 miles.