OK, I have now run a full tank of fuel through the car, and things are now fixed. Yay!
Turns out (well, best educated guess as you can't confirm) is that my previous cluster was from a C4 Cabrio. This completely explains my "optimistic" fuel gauge readings.
To fix this, and/or to code your 996 cluster to recognize the top in motion light:
Using the PST2, log into the car. You will need DME and Immobilizer codes to do this. You must tell the car it is a 996, as otherwise it will not recognize the cluster. Then, code the cluster to be a 996 C2 Cab. You must then tell the car that it is a 986 again, and load the appropriate map into the DME.
Tholyoak can probably elaborate more on this than I can as he is the PST2 master. Thanks for the help!
Patrick