Since I'm using the 817 mainly in /P, the original mike supplied with the radio is very stressed for the job. I've found an alternative one wich seems very useful for me: Vertex MH-67A8J. This mike is pin-compatible with MH-31 and is factory-supplied with Yaesu Ft-450 and also with some Yaesu/Vertex Land Mobile Radios (like VX-2100).
Is very big (almost twice that MH-31) and looks very rugged.
I've got one for few euros for an Italian ham and I've done a few on-air test on last sunday, and other hams reports that sounds identcal to MH-31.