Bonk NES shows how easily people can be fooled by a player sprite, even if it doesn't move properly. If the SMS/GG Sonic used a pixel-for-pixel Genesis Sonic sprite, they wouldn't suddenly be close to Genesis graphically.
FC Genjin/Bonk' Adventure NES is still impressive for what it is, but it's pretty far off if you try playing it or compare most aspects.
If it was close, then that would mean that SFII ports on SNES and Genesis look and sound exactly the same.