There's no known way to skip the invisible checkpoints, so this wont lead to an ultrashortcut.

However, in my loopdeeloop TAS I do a similar jump (your jump velocity is combined with the water trying to push you all the way out making it really big). This gave me a small forwards boost which saved a few frames. What's really interesting here is that you randomly started to fall through the water. If we can figure out how to do that on command sub 55 might be possible for a TAS.

Tools > Import Wii Save > select the data.bin file

As a tip from me: the timing for the Z press is a lot tighter than the A press with player 2 so you'll need to find the rhythm for that. You have 5 frames to press Z, and 44 frames to press A. The window for pressing A drops to 19 frames if you get a frame perfect ground pound but that's still fairly easy to do on reaction

As the only co-op runner I dont mind if it's put as a sub category lol. Just as long as it stays separate from single player