I went from RX6800XT to the RX6950XT and there is no difference in performance. I actually feel that my 6800XT performed better in MacOSAs you are only spoofing the device ID, not changing the VBIOS, you should get the performance available from the card as allowed by the drivers in use.
An RX6950 XT spoofing the ID from an RX6900 XT would not make any difference, as the two cards are nearly identical bar the power requirements.
With an RX6650 XT spoofed as an RX6600 XT you might increase memory speed and possibly see a larger power draw, but you would be hard pressed to notice either in real world usage.
To be honest the Revised AMD 'XX50' cards are just a filler from AMD to kid people in to thinking they are getting something new, when it is just a rehash of the same card with a slight increase in memory speed on two of the cards. The Stream Processors and quantity of Memory has remained the same in all three revised cards. So as I see things they are basically a gimmick to fill the time between the RX6000 and RX7000 series release.