Personally and a number of forum members have experienced High Pressure fuel pump failures. There is a BMW SIB (SI B 12 55 06) that deals with symptoms and warranty work procedures. The Engine management PDF file has a great section and schematic of the pump as well as a great explanation of the role of the high pressure fuel system that makes the second generation DI system possible with precise fuel metering and up to 200 bar rail fuel pressure. It really helps to understand the role of the HPP and it's complexity that obviously caused the early pumps (N54's manufactured between 6/06 to 9/06) to be prone to failure.
Before my car's HPP was replaced, I experienced two episodes that were very different from each other. The first incident at 3500 miles was a case of RPM dropping briefly below 600 at idle and engine running very very rough. Since I was at a light when it happened, I pulled over, rebooted (stop engine, restart) and the problem went away.
The second incident happened around 3800 miles. I was coming up a long incline (6% grade over four miles) and the engine felt like a Honda's. It wasn't rough, just no turbo boost. Again a reboot made the problem go away.
Limp home mode is discussed on page 28 of engine management system which includes complete deactivation of the turbochargers via wastegate valves. I believe I experienced this up that incline.
Around page 8 of the Engine Management PDF file, the explanation of Bank 1 & Bank 2blow off valves give a great insight to another major reason for lack of turbo lag beside the bi turbos and because of the DI. The main purpose of the blow off valves are to prevent pumping action and fluid dynamic problems downstream of the throttle body when driver lets go of the gas pedal suddenly, but engine management also uses them for another purpose. When the engine is idling, engine management spools up the turbos but the additional boost is relieved through the blow off valves. When power is demanded, the blow off valves are closed and the boost is already there. Viola, reduced turbo lag. This would not be possible without DI. The intake manifold downstream of the throttle body has only air in it and no gasoline, hence the blow off of compressed air back to the intake system.
There is an ongoing discussion, which I don't know of the resolution about one or two oil coolers in manual vs. auto cars. Picus in Canada is especially ticked. It is interesting that the PDF files make no distinction between oil coolers in manual vs. auto cars. However, they do talk about two gear box oil coolers for the auto as well as a heat exchanger between the radiator and primary gear box oil cooler in auto cars.