Well I only have a dual core MacBook Pro where I use Traktor and Serato, so here is what I see:
Traktor: Will use both cores, but I only have 1 core at 100%. So I do not know if the CPU is simply powerful enough and the second one is not that needed…
Serato: Seems to use both cores very evenly. Especially the F/X seem to easily profit from 2 CPU cores.
I run Ableton Live 9 on my production iMac (i7 4 Cores, 16 GB RAM) and I am using the 64-bit version of Ableton. It does definitely use all cores and runs like a charm on that machine. The new MacBook Pros should be on par with my year old iMac easily.
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