I think the jury is still out on the use of premium.

I have not tested enough to have any significant results.

The thing is indeed faster with the premium.
The percentage increase for premium goes down with every price increase, BUT, I can't prove that the mileage is better.

As for the B versus L issue, I have read that B doesn't provide stronger regen. What I haven't done is played with it in the real world to see what it does. (Believed what I read...) I'll fool around with it the next couple of days and see if I can see the regen difference on the guages the way you can on the FEH.