I didn't know the Wii Game was that faithful, makes me wish I got it back then, but at least there's Homebrew and "knowing where in the seas to sail".
I just don't think much hidden stuff (other than AR differences when triggered like you mentioned) can be done in Mario Kart because of how they're designed differently. In Toy Story you're going from screen to screen, which I'd imagine makes it easy to implement that stuff, where in Mario Kart you're moving through physical sets.
The limited ammo, I can understand and I hope that can be fixed in the future. I actually don't mind the quieter parts though, they probably made it so it's a bit easier to take in the set design and a place to fire shells if you have any.
I'll admit that I was mildly bummed out when word slipped through the cracks that it wasn't fast long before the land opened in Hollywood, but I think it's much easier to appreciate the ride as its own thing when you're not trying to compare it to RSR, especially when you consider that if it was fast like RSR, then items would have most likely been sacrificed. I get that's where it raises the question if a traditional dark ride would have been better, but it's hard for me to say; I know people that like interactivity over everything being sitting and watching. Given how the series has been a video game, maybe that's why they chose interactivity.