Just did some test with the Singh-Ray.
As far as image quality goes, I see no issues whatsoever at any stop level, so I won't bother with a show and tell here. I simply can't tell the difference.
There is a color cast however, and it goes like this. At the minimal stop level (about two stops) there is a slight warming as compared to a pic without the filter. As you move to higher stop levels the color cast moves toward the bluish side, wherein at about 3 to 5 stops you achieve something very close to parity (neutral) with the pic that has no filter at all. Moving on up from 5 to 8 stops the color cast shows to be on the bluish side as compared to the pic with no filter. Here is a screen capture that I hope shows this off, starting with no filter on the first thumbnail, followed by the filter with increases in stop value, up to going past the max for the filter, wherein all hell breaks loose. Shooting at 24mm, I'd say you start running into trouble at about 6 stops. Thats the nature of the beast. Shoot at 35mm and you can likely get away with 8 stops ..... maybe. 50mm and above, have fun, it won't let you down at the maximun 8 stops.
The color cast can be worked with in post processing to whatever degree that might be.