Shawn
It turns out that I share an office with a former ETPS tutor of yours! We discussed the subject of spiral stability and he suggests that I give it a try (when there are no passengers!).
I am not a test pilot but do have the advantage of running our Flight Data Monitoring program so will have access to the flight data recorder data, which of course includes control position information to a resolution of around 0.1% of full travel. It seems that the primary problem with spiral stability is getting the aircraft steady enough so as not to have to move the controls. It occurs to me that I can average the lateral cyclic position in level flight and again in banked flight to see what the difference, if any, is.
However one issue springs to mind immediately - do you keep the pedals in the same place, or apply pedal to keep balanced flight in the turn? If the former, there will be a roll effect from the yawed flight when banked that would tend to reduce any tendancy for the bank to increase, and this would be more powerful with increasing speed. If the latter I suspect that out-of-turn cyclic would be required to maintain bank angle. Perhaps this is where our differing opinions stem from - in my case I assumed balanced flight but in your case perhaps not?
I'll let you know what I find!