XPS 8900 M.2 NGFF Specifications?

I am wanting to upgrade to a M.2 SSD, have been eying the Samsung SSD 950 Pro, 256gb.

Does the XPS 8900 bios support booting from the M.2 drive slot?

Is the XPS M.2 drive slot a PCI-E x4 speed?  Is there a published length somewhere from the slot to the screw/post? (in millimeters).

I would hate to buy a SSD 950 Pro and then it either doesn't fit or the XPS won't boot from that drive slot.  Then it would be a 200 dollar paper weight.

Any info would be greatly appreciated, thanks!