At the point of ordering .. but YOU can't upgrade it later .. Apple would have to .. the machines aren't meant to be disassembled easily (at least the new MBP).Sure you can upgrade the hard drive.
The ram, not so sure. But really, who needs 16 gigs of ram for dj'ing. Guys get away with 2 gigs now and look at what the software can do.
Having more than 8 gigs of ram is the least of my worries.
But listen, you have never seen a more slick dj machine until you have seen a Macbook Pro with a terabyte hard drive and 8 gigs of ram.. I would love for you to just see one of my machines work.
Again .. I agree they're slick machines and if you have the spare change, go for it.