This is just about what the powertrain loses. Every manufacturer gives it's performance figues (hp/Nm/lb-ft) as measured from the engine. The transmission before that power and torque gets to the road always uses up a bit of those numbers. That was what we were talking about.
The difference that a manual greabox vs. torque converter automatic gearbox makes is a completely different issue. There too old-style torque converter automatic gearboxes (slushboxes) (basically any type of automatic tranny, including BMW's Steptronic) are usually a little slower in accelleration vs. manual gearbox with the same car/engine, and are definitely worse in a track setting since downshifts can't be controlled very well and the automatic gearbox is bad at guessing when it's time to downshift, even though they have been getting better and better over the years. They will probably be replaced by dual clutch gearboxes in the future in all BMW's (then the options will be manual or dual clutch, or in M cars, SMG.)