Oh and the clutch is fine. It's DRAG, not SLIP.
The drag is where it doesn't release quite as quickly as it should do and the transmission losses are calculated at the point that drive is released.
This is why the rolling road operator adjusted it down from 179bhp for the drag on the release.
You'll see that my Syncro also has substantial transmission losses, same road, same operator, from a couple of months back:

I tend to forget about the losses as it sticks to the road/track like a limpet, whatever the weather (much like Danny's).
How does 156bhp vs 173bhp = 7bhp?
Danny's did about 40mpg from Dover back to Ross, too. You see that often?