Well, if the genuine drivers from Intel's site installed without any modifications you can be pretty sure the chip is genuine.
Now whether it there is some other fault or poor component is another question, but nobody could easily fake the chip with it's whole driver stack.
As for why no improvements? Could be the aerials or the router. Can a XPS 15 9560 actually come shipped with that card? And if it does, did you look in the Dell hardware maintenance manual whether in that configuration it comes with the same aerials?