USB cable characteristic impedance
For USB, Zdiff = 90 Ohm and for PCI-Express and most other diff cables, Zdiff = 100 Ohm.
So they're not equal, and you'll need matching circuit for it.
It is better to take SerialATA or HDMI or Ethernet cable for your task, as they all 100 Ohm.

Cable should normally be grounded. This does not affect Zdiff in theory, but greatly improves noise performance.

(Additional theoretical note)
In general case it does affect specific impedance if conductor is grounded or floating (e.g., triaxial cable with inner shield grounded or not), however for symmetric differential cables it does not matter.