I had the opportunity to mount my 75 Lux on Damaso's M6 TTL with Rapidwinder at a NYC Meet-Up. The balance of the M-body is restored and the nose heavyness created by the 75 Lux's length goes away, even without a Rapidgrip the added 5/8 inch to the width of the M-body makes a signifigant contribution to comfort.
I would first recomend a Rapidwinder before a grip, but the ML grip is perfered over the M-Grip (which I also happen to own). Be aware that some early Rapidwinders do not have the necessary threaded hole to mount a Rapidgrip BTW.
I agree with you that a 75 Lux is a must have/keeper. Paired with a 35 Cron and you have a wonderful 2 lens long/short kit. I found that the focusing tab on my Cron falls more naturally, and the dagger/spike spreads my hands apart to provide a more secure attachment without compromising my ability to focus, when I wrap my pinky, or at times two fingers.
Although counterintuitive, I find carrying and using the Lux with Rapidwinder and Rapidgrip less fatiging despite the added weight when I carry this rig all day walks in NYC. The 75 Lux also feels more ergonimic on M8's and M9's with their thicker bodies. Perhaps this coresponds to the price drive upwards since the M9 release.
Cal