Kevin
The AI is used for the AP. I have a 2 1/4 T&B. Room is the problem. There are some other alternatives coming along and echoflight will be offering a AI display using a seperate sensor. Midwest makes one as well......... even Garmin is offering a "form" of display in the new GPS.
Soon VACUUM AI's will be a thing of the past. The electronic gyro or rate "platform" and resultant cost has been the hangup. In the meantime the vacuum pump will continue to drive the boots and AI
I think you can replace an T&B with an electrical horizon with authorization.
I now have my moving map in my scan which is an early predictor for an AI failure ...... heaven forbid. I caught an HSI failure that way before ATC caught the problem. This occured shortly after takeoff on the climb. I continued on destination without a problem.
Bob