I really didn't want to get into any pedantic discussions, just in case I sounded pompous and condescending. I tried to keep my question simple, in the hope that someone who was actually flying FSX on a widescreen monitor would instantly understand what I was asking, and be able to respond in detail.
I actually have three 22" widescreen monitors on my flightsim machine.
I also use one 22" widescreen and one 22" 4:3 screen on my development machine.
I've run FSX on my fs machine with the fsx.cfg entry set both ways, and to be perfectly frank can hardly tell the difference because the effect is so subtle... ;D
In any case, as I stated above, that entry has no effect whatever on the 2d panel system. That is the only part of FSX that won't run properly "out of the box" but will require user intervention if it is a significant matter to them.