Well, the 3 B/B+ can be shortened considerably, there are hack projects for that. The battery is definitely overkill as well. My battery life is easily 8hrs+ with a 10,000 mAh battery. I can use a prefab sound card from Adafruit and lose that bulk too. The display won't be a problem it is actually very thin and the same footprint as the Pi. However, I really love the feel of a full size controller, and I don't think I'm stuffing that combo in my pocket any time soon.
However, when I finalize the build, I'm going to be ADDING weight in the form of RF shielding of the sound board and Pi/display. The sound board is picking up too much crap no matter how many filter caps I throw in the circuit.