Alright, just finished the game today, took around 6.5 hours. I found the pneumatic transport room at about hour 5 and paused to go back to the previous areas and clean up, and I was quite surprised to see that I was at 100% when I saved before the final boss to check. I feel like that would not have happened in a full-on Nintendo game, as I did not go crazy power bombing every room and whatnot looking for secrets (I've played Super Metroid three times and never got above 90%).
Which kind of feeds into my overall assessment of the game: It's a really cool project and a clever adaptation of Metroid II, but in the end it doesn't quite get to the level of Fusion or Zero Mission. I was pumped when the new stuff started showing up mid-way through, but it becomes clear that most of the additions are kind of half-baked or just inconsequential. Like, I was pleasantly surprised to find the research ship on the surface, but it turns out to be tiny and just have an energy tank and a forgettable mini-boss. The power plant was probably the only addition that I thought was fully effective. And aside from the new stuff, it's chained the original's structure which doesn't hold up that well in terms of general level design.
Definitively worth checking out, but I also wonder what I would think about it if i wasn't constantly cross-checking it against the Gameboy game, which I played earlier this year. I also feel like Nintendo would have put out a superior adaptation given the limitations of the base structure, with something even more off the rails than the suit-less section in Zero Mission. Like, the queen wouldn't actually be the queen, and would unleash another round of 'troids into the planet or something for a surprise back-half of the game.