I Never had a problem using Mid-13 Air with i5 1.3Ghz, 4GB using traktor in DVS mode (~30-40% CPU), never gets freezed music and other glitches, instead of Windows laptop with more powerful i5 and 8GB of ram, Rekordbox uses far less of system power in library mode and even when you are playing with controller. You just need to switch from Win to Mac. And you will se how computer helps you to get the best music experience
As for me – i don’t like the idea of modular setup… So, i need all-in-one
About headphone/master out. Mixtrack pro, for example – two stereo outputs on back – i’m used it with external mixer. And one of channel mirrored to the front with headphone jack for internal mixing. Reloop Contour interface edition utilizes the same model – 4 outputs on back and headphone jack in front – copy of first channel. So i don’t think that on K2 is SLIGHTLY different channels on the front and back, but i didn’t seen that disassembled.