Well, i can definitely hold Setsuo Yamamoto and Yuki Iwai for candidates.
Setsuo always seems to use these effect enhancers with his tracks, every game(or almost) he has done he does this, RMX, X-Men MA, and maybe SFZ1/2. Guile's theme uses these same effect enhancers the setsuo RMX tracks and X-Men MA tracks have. When i sample swap Guile's theme with X-Men MA samples, it even sounds very close, the only difference is a few things altered to suit a particular sample, or else it would sound awkward.
Yuki Iwai, well some of them have many things in common with how the RMX2 songs were put together, which Chill Penguin from RMX1 uses, an alleged Yuki song, which basically puts out further proof she did that song. Anyway, i noticed she always seems to have her instruments fade and often has this sped up hat sample(hard to explain unless you extract the music and look at how it was put together) that nearly every Yuki SFC song has. If there is any song that i can say is a likely yuki song, it is T-Hawk's theme, the strings have that same fade effect and the sped up hat is used in there. Dee Jay's theme also uses these effects(except it's for the brass rather than the strings as it has no string sound in this song). There are a few other ones that do this, but that's not the point, lol, just making a point here.
If syun did any, i have found a few that use that same echo effect heard in the arcade version, ones like Cammy's theme uses it, M.Bison/Balrog's theme, Zangief's theme and few others, if this is correct, then it seems with the SFC he uses a very hard heavy and strong vibrato effect. I also heard that he was involved on the SFC version of Muscle Bomber, which not only sounds very similar to SSFII on the SFC, but it also uses that same heavy vibrato effect, which is the case with every song in there, wheras SSFII does not do this. It also uses a layering effect similar to Zangief's SSFII SFC song.
As Isao Abe, i am not totally sure i got him all figured out, though i get the feeling he handled Sagat's theme. And it does sound put together in a similar manner to that of the first SFC two, just altered to fit the samples, of course.
And the hats sort of sound like guy's SFZ theme, i don't know. I also suspect he uses instrumental blending, he does so with Sagat's theme and Ken's theme uses blending, so i guess this is his work?
Well, anyway, if i am going to use an unofficial breakdown i am going to guess this for their stage themes.
Ryu: Syun Nishigaki
E.Honda: Isao Abe
Blanka: Isao Abe(?)
Guile: Setsuo Yamamoto
Mike Bison: Syun Nishigaki
Ken: Isao Abe
Chun-Li: Isao Abe
Zangief: Syun Nishigaki
Dhalsim: ?(Isao Abe?)
Sagat: Isao Abe
Balrog Fabio La Cerda: ?(Abe?)
T-Hawk: Yuki Iwai
Fei Long: ? Definitely not Syun as he would never use that echo for the vibes.
Dee Jay: Yuki Iwai
Cammy: Syun Nishigaki
Lord Vega: Setsuo Yamamoto(Same Effect as Guile's theme)
Just my guesses.