Concerning engine brake-in: on all the cars that I've broken the engine in (e39 540i/6, e46 M3, e60 M5) I've always pushed the car, putting the engine through its paces, but never redlining the engine. I've also maximized engine braking, which is what most engine builders will tell you is the best way to break in an engine, resulting in better seated rings, etc. My cars have always dynoed strong, towards the upper range. (these were with stock motors)

So I see no harm in driving in DS mode during break in. Engines that have been pushed, but not abused, tend to be looser and make more power. I have 68k+ miles on the M3 (with a lot of track miles)and it has been supercharged for the last 5k and shows no sign of wear or slowing down. In fact it recently put down 406 rwhp (which I hope will be even higher with the new rev limit).

Concerning gas mileage: yes, running the motor up in revs will burn more gas. No question. On the track my M3 gets ~6mpg and rarely sees anything less than ~5k rpms.

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