Hello! I am not aware of either of them having used the high impedance model of the FV500. Also, only Ed uses the FV500 as an expression pedal (using one with his EHX HOG and one with his Diamond Memory Lane 2), but Jonny and him both employ them as volume pedals. The impedance has no effect on its function as an expression pedal, only as a volume pedal.
The high impedance version is designed to work better when a guitar is plugged directly into it, while the low impedance version would cause more treble loss with a guitar plugged in directly. However, placing a buffer or buffered effect in front of either volume pedal will negate treble loss for both, but the curve of the low impedance is theoretically better for the “active” buffered signal. However, the difference is fairly negligible, and the two can be used more or less interchangeably.
Since Ed and Jonny have their volume pedals placed after buffered effects, it makes sense that they would use the low impedance version of the pedal. That said, Jonny has used the high impedance version of the Boss FV300 for many years after buffered pedals, perhaps showing that he does not consider the difference to be all that significant (or perhaps he even prefers the different volume curve).
However, the FV500L also works in stereo, though neither Ed nor Jonny have used it in that capacity..