Interesting. My volume definitely adjusts on the fly. Gets lower when stopped or at low speeds, and then much louder at higher speeds and higher ambient noise.
And I've tried all kinds of different power-on sequences and find that if I turn on the SRC first, and then turn the ignition on within a minute or so of turning on the SRC, I get the most consistent pairing success with the bike.
Jeff - is that the overall volume, or just the GPS?
My NAV-III has a speed/volume auto-adjustment, and you can set it for different volume increases with speed.