Here are my results. There is very little to no difference between Wii and Gamecube component video. Imperceptable in fact.
Here is Wave Race Blue Storm:

This image was captured via component video cables in 480p on both systems, recorded at 10-bit YUV Uncompressed (no MPEG steps) and uploaded as an uncompressed PNG file. This is 100% normal size of the video, no scaling.
Here is a 400% scaled version of Wave Race Blue Storm:

Same techniques as above, but this was scaled 400% using nearest neighbor, no scaling interpolation to muddy things up. Again an uncompressed PNG.
Bottom line: People pay too much for GameCube Component cables when the Wii gives an identical result for all intents and purposes.