Compnerd,
The wildcard here is that you have client-aware on, so constant adjustments cannot be made. If you want to put a "floor" on the power that access points receive, you can certainly do this with the arm min tx power setting. In your circumstance, however, you possibly have too much coverage, so your access points will always be at the Min setting because they will see too many access points on the same channel.
For VOIP with handsets on 5ghz, we have deployed in some hospitals at every 50 feet, and the access points between 12 and 18 and gotten good performance. Unless you have pretty good attenuation, like concrete walls, your access points might end up on the lowest transmit power regardless of the version of code. Long story short, the change in behavior may be notable, but only if it creates coverage holes. You can uncheck client aware during the day to see how they settle when the full compliment of clients are in the spectrum. It normally does not create alot of disruption. 9 is the lowest that you should go, because any less, will not match the output power of the clients that are connecting and possibly cause communication issues. All your access points at 9 after allowing them to change overnight means that ARM thinks your spectrum is crowded -- just my 2 cents.
You can set the transmit power very, very low, but most if not all clients are at full power, and STILL can interfere with each other and other access points... So the lowest common denominator alot of times is client transmit power.