XBMC is compatible with 4.3 if you are confident enough to install one of the semi-nightly builds that are in development, they are less stable but are a good solution if yiou want 4.3 to run on your appletv 2....
I am in the same boat, i still have appletv 2 on 4.2.1 running the official build of XBMC, i would love to update for AirPlay support but there is no definitive answer if the nightly build run on 4.3.1 yet and if the tethered JB works on 4.3.1 either.
I am happy with tethered as i have a macbook to boot if i need to power down my appletv.