I've heard a few times that the Wii U provides faster load times relative to the Wii, which may be useful for multi-stage/sports runs. How much is the difference in load times and would it be worth it to buy a Wii U just for that purpose?

i mean thats always kinda personal depending of the money it costs in contrast to your economical situation, if you are going to make any other use more of it, the level you have in this game... but well, in general, it isnt realy that much of a diference in comparition to the normal Wii after all, idk sure about this game but thta normaly isnt something to care that much if you are not in a top level of the game i mean maybe you can get a 2nd hand wii U for only 20/30 €, so if you feel like doing so anyways

Hey, old fart here. I think we calced it being like 1 frame faster per load time (something like that, ask Tyler). I feel there is a little more time save in some other areas, but in specifcally load times that is what was agreed upon (back in the day at least).

Whether it is worth it or not is just dependent on how optimized you want said run to be. I got one used for $89 and I got its mileage out in testing stuff and the few runs I did at the end of my time.