I'm not sure about the precise arrangement of drive system on the 330e. It may be true that running a petrol at it's most efficient speed to generate electricity might be a better option than running it at, say, very low speed crawling along in traffic. Certainly you can do this in Ampera because having an epicyclic gearbox with inputs from ICE and large electric motor, as well as another electric motor/genny where the flywheel would be, this car can play lots of clever tricks like running petrol at optimum constant speed feeding some power straight to wheels (max efficiency) as well as charging the battery with the excess mechanical power going to the small electric motor(in generator mode), and using the large electric motor to provide the variable-speed needed as car accelerates & slows down. Very clever stuff indeed.
However, I don't think the BMW is anything like as sophisticated, and I don't know if it's possible to decouple the ICE from the wheels in such a way as to charge battery optimally with ICE at constant speed, while also driving the car electrically along the road. If this is not the case, then the ICE speed is going to be coupled to wheel speed via 6/7/8-speed transmission, and it's going to be under varying load in all probability at the car goes up & down a bit. So it may be a lot harder/almost impossible to guarantee optimun charging.
On a seperate area of efficiency for OP's interest, one technique to maximise electrical efficiency is to avoid heating the car electrically. The technique is to heat the cabin really hot (25C or more even!) when running on petrol, so using the waste ICE heat. Then, turn the heating off, fans, a/c off etc, and switch to electrical driving for a few miles. The cabin will cool slowly, and when your toes start to go numb, go back to petrol & warm the car up again!
This isn't always appreciated by passengers in the car, unless done gently & subtly so they don't notice! With a bit of luck they fall asleep when the cabin's toasty warm!
