We would not recommend the use of a higher supply voltage than 9V.
Higher voltages will increase internal power dissipation, and any peaks much above the normal could potentially (pun) cause damage.
In addition, the powerfail detection circuitry (used to save changes) is calibrated for use with the supplied power supply and may not work properly at other voltages.
Our problem with power supplies is that the unit was designed to use the supplied power supply. As your letter demonstrates, there are an endless number of alternative options, and we cannot rigorously test them all. We are looking at some of the more popular supplies and will report back in due course.