I finally decided to snag an s-video cable for my Super NES. I checked all the used game stores around me, assuming they would have something intended for the Gamecube, but no luck. I ordered an official Nintendo one from Ebay. There are approximately one billion of the crappy composite/s-video combination cables listed, so it wasn't easy to find.
I have used mine with S-Video, and the only issue is the connectors on some cords with thicker jackets on the a/v connectors don't fit into it very well, it's a tight squeeze for my Xbox, for instance. But as far as actual quality, it's been just fine for me over the years.
Mine also has an RF modulator built in, but I have never had a reason to even try that function out. For that function it requires an AC adapter (included) but nothing else requires power.
I just got an official Nintendo s-video cable. For some reason, the red and white cables are reversed. On every other cable I own, along with the inputs on the TV, the order of connections is yellow/white/red. This cable is s-video/red/white. I guess I need to find a game that has distinct left/right sounds so I can test that the cables are wired correctly.
I tested the sound and it is wired correctly. I got this cable from Japan, but I can't imagine that Japanese audio inputs are reversed. I guess the s-video cable is just like that.
So I did a few comparisons with various games like Castlevania IV, Super Mario World, and Mega Man X. At first I didn't notice much difference. But when switching between the two, the difference is clear in the details and colors seem much more accurate. However, the difference isn't huge as with the PSone composite to s-video upgrade.