So I noticed this new WR run (even though it's 8 years old) has a potentially useful trick in it. Skip to 1:57 and look at how he's shifting to retain booster speed. It could just be a visual trick but I'm not sure, could be something interesting. I have no idea where it could be useful but I'm going to attempt it on the US cartridge.

(Current WR?)

I've done some experimenting with TAS and I recorded 2 separate TAS runs. One using the R3R5 route, and one using R2R4. I was surprised to see that R2R4 was faster, but going by the segment times the optimal route APPEARS to be R3R4. Here's the clips. Both of these were done on the US version of the game.

R3R5:

R2R4: