I used the same board and similarly power from the regulated 5v line on the raduino. It also offboard and just inline with the mic line.
75k for compression and 1k for gating and 100k pot on output. At full output my audio was readable but distored based on the one audio check qso I had before I turned it down a bit.
I put a larger heatsink on the irf510 too to cater for the higher average output as the stock one was getting warm where previously it didnt.
A sideeffect of the mic being on all the time is that there is leakage through to the speaker and it causes some feedback if the mic is within 2 inches or so of the speaker. Not a big problem.
Sorry all for the off topic content!