JF makes a very good point, particularly in light of Shockwave's more recent comment.
The overspeed / overbank condition (which I trust you haven't tried with Pax on board) is probably what I'd know as a spiral dive. If I were dealing with a "conventional" aeroplane, I'd associate high spiral stability with a tendency to lock into this mode - this killed a fair number of WW1 fighter pilots before it was understood. So, what you're saying is inconsistent if there were no AFCS in the loop. I think JF is probably right, the AFCS is probably doing most of the work. If I were Chief Designer on the A320 I think I'd be content to make most of my apparent (i.e., as seen from the cockpit) stability artificially. I suspect that whatever the spiral stability really does, you have a system designed to display apparent moderate spiral stability in small perturbations and low spiral stability in a spiral dive.
Now if only I could work out how to do that with reversible controls...
Freelunch, be careful about using the word stability. It is a very complex subject to wrap up in one word. Spiral stability is a function of the RATIO of lateral to directional stability, both of which you need. The efficiency drive in the Airbus is pretty much all about longitudinal static stability, which is a rather different beast and nearly, but not-quite, unrelated.
G
[ 24 November 2001: Message edited by: Genghis the Engineer ]