So a little bit of googling around lead me to this great interview with Christian Whitehead, which lead to the discussion of his iPhone Retro Engine and Retro Engine Development Kit. Basically he took the PC port of Sonic-CD and was able to extract the levels, sprites, and also the scripted logic. Then he was able to take all of that and build his own re implementation of Sonic-CD.
Which in its own is interesting as the PC version of Sonic was done via some translation as mentioned by Neil Harding here.
… I had to write a 68000 to C converter for Sonic 3D (from the Genesis to the Saturn & PC)….
I would imagine that the ‘unpacked’ work of the PC version would lead to something far easier to reverse, then say the original 68000 code, although that too has been done.