Check out the Current World Record for Mushroom Gorge NOW. (Youtube hates me now, so no video here, download the WC Mushroom Gorge Time Trial) It seems he used the mushroom in the beginning to go back, using the walls somehow, words cannot describe this...cheating shortcut
I'm quite disappointed at how people do race like this. Though, I'm sure that Nintendo will make sure that this is cleared up, like they did so for Grumble Volcano. (I actually was able to go around the mountain, at the start of the race, while using a mushroom but soon, the walls of Grumble Volcano were raised somewhat higher so I really couldn't do it again. I never used the shortcut anywhere, though I just enjoyed trying it).