I'd like you to explain what you are talking about because I'm not a casual player of Street Fighter and I don't see much of anything fundamentally different. They changed the sounds a bit, added assets, it counts combos and reversals. Does the whole game need to be rebuilt to do that? I'd be surprised if anything but the first of the CPS versions were ground-up constructions.
Well, I don't think either of us has enough insider knowledge to truly assert one way or the other. I'm pretty sure, but I don't really have anything to hold up, which seems to be the same for you. So I guess we're just going to have to agree to disagree pending any actual evidence either way.
So...even though there was no reason to build the game from the ground up, and no significant appreciable difference in sound, graphics, or gameplay, you are for some reason just assuming that the game was rebuilt from the ground up for Super...well, OK.
My point is that
if a ACD version of SSFIIX was to be made by NEC or Hudson you wouldn't
have to do that (even if it was done with the CPS versions, which I doubt, see: Occam's razor) and you wouldn't need to port anything from a SNES game (the source for which NEC would't even have access too). Building the game again or porting it from SNES before adding the Turbo elements...those are just needlessly complex directions. There isn't any logical reason whatsoever to do it when %80 of Super Street Fighter II Turbo already exists in the form of SFII'.
Step 1: Re-open the SFII' project
Step 2: Convert project to an ACD (hardest part?) while adding new routines for combo counting and reversal bonus.
Step 3: Add new characters, BGs, endings, moves, and color tweaks.
Step 4: Replace some sounds
Step 5: Compile image, ship, and profit...or more likely, loss
This is, more or less, what SNK did almost every year when KOF came out.
Maybe you guys have forgotten that these are all versions of Street Fighter f*cking II. It didn't change that much at all during those 5 games. In fact, the upgrades to the earlier games were done by swapping a few 24 pin ROMS and keeping the vast majority of the board.