I don't really understand the point of this project to be honest, as why have a 1:1 recreation of the game if we have 2013 remaster? Widescreen? We already have a patch for the rom and a custom version of the emulator. I would understand if it was made to show off what the framework is capable of, but no. Besides that though, yeah, it's as faithful as you can get if we're not counting the speed caps that nobody liked. If you also want to get more faithful, the spinning Spike balls in Spring Yard are not fast enough in some places. Also, I don't know if it's just me, but the brightness feels too high. I'm sure it's not my monitor as I played Origins not long ago and can see the difference. As for the bugs I've encountered, the invincibility trail will stay on if you go to special stage. (maybe it's like in the original, beats me). Good, yet unnecessary attempt at recreating Sonic 1.
The invincibility trail bug was fixed in the patch that was released a few days ago.
There is no problem with the brightness, the colours you see in-game is how the game is actually rendered by the hardware. The only emulator that currently does that is BlastEm