You're asking if you should use mid-grade or premium in a brand new BMW, a BMW with a twin turbo I6 no less? If I had just bought a $50k+ high end car, I'd probably use premium just out of principle alone.
It's true that premium fuel has less energy than regular, but what's much more important is that premium fuel has a higher anti-knock index and your car's engine was designed for premium. There's really no benefit from using regular in car that was designed for premium. Best case scenerio, you get less power and less fuel economy (since the knock sensors will reduce timing, and the ECU will fatten up the fuel mixture to prevent damage, using more fuel in doing so). Worst case scenerio, your knock sensors fail and you destroy the engine. Your warranty will not cover anything if you use a fuel with lesser octane than required.
For your specific car. If you look on page 241 of your owner's manual. It says the following:
Super Premium Gasoline/AKI 91 or AKI
BMW recommends AKI 91 or 89.
Gasoline with lower AKI
The minimum AKI Rating is 89.
If you use gasoline with this minimum AKI Rating,
the engine may produce knocking sounds
when starting at high outside temperatures.
This has no effect on the engine life
So according to BMW, You can use 89 octane fuel, but there may be some minor side effects. 91 Octane is recommended. Personally with a turbocharged engine, I'd spend the extra 2 bucks a fillup for premium. You can try the 89 if you want and see if you take a hit in fuel mileage. Even a minor drop in mileage will more than offset the added cost of using premium though.
Note: It's also possible they require 91 for the turbo engines and 89 for the N/A engines. You might want to check with the dealer about that.