If you did'nt authorize it, return to the dealership and get your money back.
I don't drive an X everyday and I can't speak about the ride of one shock versus another but I know there are people on here that can.
I can tell you what it feels like when shocks wear out.
The shocks produced for Nissan by Bilstein are made to OEM standards dictated by Nissan, what they are I don't know as that info is unimportant to me.