This mod was submitted by Pete PE1MHO.
CAREFULLY open the microphone.
(There's a little spring in there that will fly out and get lost in the carpet, never to appear again. )
Remove the circular metal plate that holds the electret capsule in place, and GENTLY pull out the capsule.
You'll find what can best be described as a "Q"-shaped rubber gasket - remove this and throw it away.
Reassemble everything - that's all there is to it.
You will now have a microphone with a bit more punch, but most importantly the audio bandpass is a lot better.
The difference as measured on an audio spectrum analyzer was quite significant.
I'm not sure if this helps for the Mk II and Mk IIG - maybe Icom have already adapted them....