In fact I think that, judging by the beta screenshots, SFA2 wasn't supposed to be what it turned out to:
Notice how big the sprites are in the beta. I think that it was until later in the development cycle that Capcom noticed that they wouldn't get by with 32Mb (or 48Mb for that matter) and went with the S-DD1.
As for the Genesis port, well, I don't know. I'm not really familiarized with Sega's 16bit hardware so I don't know if it could pull it of. ROM size would be the main issue although a Sega CD version sounds interesting. It would have been faster, that's for sure. Still, I don't think that Capcom would have made a port even if they could. The Megadrive was actually dead when SFA2 was released but the SFC still had games coming out (the last SFC game was released in 2000).