So, after giving this a shot in both Vanilla and Mystic Realm, I have to say, I'm really impressed with this. It's got a lot more extra than I was expecting it to have, and some of these additions (you know which one I'm talking about) really were worth the development time they undoubtedly took.
The spritework here is also really great, even down to giving Super Shadow the magenta flames for his shoes. The ability to color those flames is also a very welcome touch that I appreciate for netgames quite a bit. If I had to make one critique for the sprites, I'd clean up the front, front diagonal, and side angle skating animations a bit more to make them more fluid and on-model, but besides that, it's pretty much perfect.
I've had 0 desynchs in netgames due to Shadow. He's fast, fluid, and fun. He's also very well balanced and fits right in with the vanilla characters. I also really appreciate the effort to keep him in the "Classic" style to fit in more, such as the black eyes and lack of modern sounds/ability-style.
Overall, really impressed, and I'd be really happy to see Shadow, alongside Metal Knuckles and Tails Doll, as Vanilla characters in future versions.