Being digital prevents degradation of signal, but that does not mean the original picture is of higher quality. What's going on behind the scenes in the hardware is also an important determining factor in the quality of the picture you'll get.
The Gamecube had a digital A/V out port that the component cable plugged into; the cable had an embedded ASIC that converted the digital signal into an analog YPbPr signal. The Wii, on the other hand, did not have the same hardware resulting in a lesser quality component output. So how the Wii U hardware works to create the image and send it to the HDMI port would be critical in knowing if it actually gives you a better picture than the Gamecube's component cables. Or, of course, if we had somebody actually test with real gamecube component cables and Wii U over HDMI, we could just use the eye test.