I actually grew to like the wide-ish normal lens. It's actually quite sharp, particularly when stopped down to 5.6 or smaller. I've done quite a few stunning 13x19 prints from shots I've done with it, particularly the Kodachromes. It also performs very well when wide open or close to it. The only times I've had any issues was when I was really pushing it, low light scenes with bright light sources in the field gave "UFO" flare artifacts and crescent-shaped overexposed highlights toward the corners. Example is here, and this was shot with the Mamiya (above), but is somewhat typical of the GIII under the same conditions.
http://www.demare.me/gallery/index.php/757640-R1-031-14_014
If you're referring to my recalibration, I wanted to be able to more accurately shoot 1600 film in it. I did a careful two-point calibration using laboratory standards (LOL, a "sunny 16" outdoor scene and a low-light bathroom wall with the overhead and vanity lights on dimmers) and it killed two birds with one stone, bumping up the high-end ISO to 1600 and compensating for the "wrong" battery. I wrote up the procedure on RFF at the time, and I may have posted it here as well.
Here is the link to the RFF post. Go about 1/2 way down on the page for the two-point calibration with the photos showing some details.
http://www.rangefinderforum.com/forums/showthread.php?t=25305
Here's my opinion, and to be honest, for as long as I've had the black one, which I assume is factory calibrated for the original battery, I really don't know exactly how much it's off with the new "wrong" battery.
However, I stopped and did a quickie non-scientific test outside, using the same "laboratory scene" as cited above.

With the black GIII (I assume factory calibration) with the "wrong" battery, at ISO 200 and 1/500, the meter says between f/11 and f/16. Comparing this to the Pentax MX, the Pentax shows f/11 at ISO 200 and 1/500 using the normal 50mm lens. Therefore, under these particular conditions, the GIII shows about 1/2 stop underexposed relative to the Pentax.
There are many variables here. I do know that both cameras generally expose well. Both are well within the same ballpark and I would trust either to do a close to perfect exposure with negative film. My hunch is that a shot with slide film would be well within range too, but for an important shot, I would bracket one over and one under to be sure.
Hopefully this is helpful to you.