wrote:
Seems that the best redundancy is a rate based autopilot (read STEC).
Since it is based on, and runs on, a turn indicator the issue of
whether yo have a good one is solved. However I still have an electric
backup vacuum pump in my C172.


I really believe this. The TC won't tumble. It comes down to two things:

1. If you have the presence of mind to engage the AP if the AI goes south.
Of course, in most IFR situations, the chances are good the AP is already
on.

2. Although the standard "ethic" says that engaging the AP during a loss
of control is a good idea, I tend to doubt it. The STEC has limits on what
control pressures it will tolerate, and the result can be for it to
simply disengage if it cannot overcome them.