Exactly, why are Apple limiting the nMP to 64GB when their competitors have 512GB? Users of these machines aren't wanting something that looks nice they want performance, scaleability, reliability, application compatibility, etc.
I'd quite happily ditch the second GPU on the nMP in favour of a second CPU, extra RAM slots. The ability to add a second SSD wouldn't go amiss either.
You assume the HP is a direct competitor. Can you buy the Z840 at Best Buy?