I'm probably the exception to the rule in that I purchased the Range AFM disabler when I bought my 2019 GS, used it for a year and then sold it. I just found that driving in manual mode 99.9% of the time had the same effect as using the Range device. The remaining .01% is driven in full auto mode just to check for "shudder". So far, so good.
The unit does exactly what it claims, it prevents the A8's Active Fuel Management system from kicking in regardless of what drive mode it's in and it does it extremely well. The later versions do not add parasitic draw to the battery and they are easy to remove when taking the car in for service. Just remember to do so about 200 kl's prior to the appointment as there is a very small amount of trace code that has to dissipate from the ECU. Dealer probably won't check and it shouldn't affect the warranty anyway but removing it will eliminate any possibility of that discussion. Also know that by having the device occupying OBD II port constantly GM will not be able to scan your system and, as such, they will not be able to provide the normal details found in your monthly report. Again, no big deal.
The Range device will not change the performance of the vehicle. People have stated that the A8 shifts smoother, faster and I've seen one report of cooler trans temps. The device doe not have the capability of doing any of these things but it will certainly disable the AFM.
I see that the very latest units are also effective at disabling the newer Dynamic Fuel Management, or Skip Fire, systems, didn't know that.
Good luck
Ross