The resistor that powers the Jfet in an electret mic can feed power supply noise or even cause oscillations if a battery is used unless the resistor has an RC filter feeding it. I use 1K and 100uF from a 9V battery.
The gain of the Jfet is higher when the resistor powering it has a higher resistance. I have not tried using a high impedance current source which might cause the gain to be so high that the Jfet causes clipping.
There is a modification of an electret mic that changes the Jfet from the standard common source with gain to a source-follower with a gain of only 1 to allow a higher sound level without overload distortion.