The cable isnt the limiting factor, its the console and how its designed, and most likely the bandwidth capability of that video encoder.
On a TV, the sharpness adjustment is a bandwidth limiter, reducing the bandwidth will soften the image. When using RGB, the display device very rarely has a sharpness adjustment on the RGB channel, and the bandwidth is limited by the design of the set, in most cases far exceeding anything youll ever require for such an extremely low resolution.
The higher the set's bandwidth, the more capable it is at resolving higher resolutions. There is a hell of alot more to it than the tube, and this is one area where the NECs really blow the Sony PVMs away. At the lower res its not an issue, so it comes back to the quality of the tube and the magnetics driving it.
Having said all that, youll probably find the SNES wont look much better if at all on a PVM than it will on any other decent CRT that has an RGB input such as a domestic grade Trinitron ( provided both are correctly calibrated and magnetic alignment is as good as it can be, they really should look identical ). Yet if you use a Dreamcast with VGA, the difference will be quite easy to see.