I agree Scott that this bike is in need of a nineteen inch wheel, but in the thread you are speaking of, it was really never tested for any extent. My bike flexes excessively in the rear at highway speeds. It's readily apparent. A nineteen will help, but I think it only will mask the problem a bit. I am certainly willing to try, is there any G-5 owners out there in the NY/ NJ area who can help out? As for my dealer? He is no longer a Dealer for reasons unrelated. I'm hesitant to drop the coin and go through the labor of the conversion, only to be frustrated.
As for the suspension adjustments you mention, they have been tried. The bike only stabilizes when tucking in, leaning forward, and reducing speed . In other words, holding the front end steady, while the rear end stabilizes. This is can be brought on by any turbulence, the higher the speed, the less turbulence it takes to induce. A small bump in the road, a gust of wind, a passing truck,etc. At speeds below say 50mph, it's not an issue. And think about it, a couple of minor suspension adjustments or 1" wheel size difference should not make a drastic effect at 55-60 mph. If it does.............
Stability on a bike should increase with straight line speed, unless the geometry is way off, or there is another factor upsetting the straightness if you will. I don't think the geometry is that far off.