SRAM has evaluated this and claims the results are inconclusive. So much for experimental data: what's a model show?
I spent several posts looking at a drivetrain model. I ended up with the following:
Ploss = (K / L) P (1 / Nf + 1 / Nr) + C T0 K (1 + Nf/Nr + Nf [ 1 / Ndt + 1 / Ndb ] ) + Kd C Nf [ 1 / Ndt + 1 / Ndb ] ) / 2
where I define:
Ploss = power lost to drivetrain,
Nf = chainring teeth,
Nr = cog teeth,
Ndb = bottom pulley teeth,
Ndt = top pulley teeth, and
C = cadence.
I used the constants:
K/L = 0.265,
T0 K = 0.322 J.
A key parameter is Kd. I previously derived:
Kd = 94 mJ/rev for Shimano Dura-Ace
Kd = 2.4 mJ/rev for CeramicSpeed pulleys
or using data measured by Mark Kelly:
Kd = 37 mJ/rev (standard bearings)
Kd = 6.4 mJ/rev (high quality steel bearings)
I assume everyone uses the best ceramic bearings before even considering going to a Berner modification. So I'll use Kd = 2.4 mJ/rev.
The savings are calculated from the loss equation:
ΔP = C T0 K Nf ( 1 / 11 ‒ 1 / 15 ),
yielding 0.46 watts for the 39-tooth ring and 0.62 watts for the 54 tooth ring. Bigger ring at the same cadence means the chain is moving through the pulleys faster.
Are these savings worth it? I assumed 300 W steady power, so the savings is around 0.2% of total power with this model. Of course, all of the savings are lost if the Berner pulley doesn't spin as well as your 11T pulley, or if you don't keep your drivetrain meticulously clean you'll squander any savings from this hefty investment.
But it does appear the benefit is real, if small.
Of course if the pulleys are clogged with dirt, things may be different. In the limit of an infinitely-mucked-up chain (T0 very large), the 15-toother saves 10% of lost power in a 53/23. So maybe it's the option of choice for cyclocross, mountain biking, or other races in the mud.