It's a mightily impressive engine isn't it Robi? The best I've managed (on a run from Dewsbury to Milton Keynes) was 44.4mpg average from leaving the petrol station to arriving home. I agree with you completely, it's docile, quiet, refined and (incredibly) efficient when you're bimbling along but when you put your foot down it's offers incredible performance.
I use my car for the same purposes as you, lots of business miles up and down motorways and lots of average speed checks & speed cameras. Hardly a journey that lends itself to driving like a tool! I also have the view that my licence is crucial to my livelihood and I'd therefore rather hang on to it!
The 340i / M140i use the newer B58 engine that features a number of efficiency improvements and therefore should be slightly more economical.
Absolutely spot on.
Generally speaking I think the balance between performance, economy and all round usability for everyday driving is what makes the car ideally suited to so many people. A colleague said to me that 40 mpg after a long motorway run would make him feel sick (he drives a diesel 308 SW). He had no idea the M135i has a 3.0 turbocharged 320 bhp engine.
Will I ever get bored of the sound? No, of course not