After the CES has ended, Nvidia has announced the newest mobile workstation GPUs. The new Quadro-GPUs will be used in upcoming workstations, unfortunately, not all of them are based on Pascal.
http://www.notebookcheck.net/Nvidia-New-Quadro-GPUs-for-mobile-workstations-introduced.191100.0.html
I beleive that the disparity between XPS 15 and Precision 5520 is because of the rated TDP for whole system. It's somewhere around 100W for CPU+GPU combined, and if I can recall correctly, Precision uses full 45W CPU parts, so the process takes that extra 10 watts which are allocated to GPU in XPS 15.
Thats not true, both are equipped with full-voltage 45W Quad-Core CPUs.