Have tried XTR 12 using both Shimano and SRAM GX cassettes.
The GX worked from a shifting point of view although was a little noisy under load most likely due to the chain. The shift speed on the XTR cassette is what really sets it apart from SRAM though. Lightning fast whether under load or not, up or down. I tried shifting into an easier gear midway through a standing effort around a switchback corner and it just did it, easy as pie.
I’d go as far as to say that if you aren’t using the Shimano cassette and chain you might as well just use SRAM and be done with it as you are missing the biggest benefit of the Shimano 12 speed in my opinion.