I believe pre crisis supes was written inconsistantly enough to be waaaaaay too much for sephiroth... Cuz he moves a good amount beyond lightspeed and stuff. Don't make me go to the "punch him once before he can think" or "chuck him into the sun" arguments.
I'm just not seeing Sephiroth doing much against Supes. Unless this is RPG style, and he equips the materia that always makes you go first (forgot the name) and blasts Superman with magick. And that still depends on whether Superman is allowed to sneeze during Sephiroth's turn or not. And does something moving beyond lightspeed cancel out that materia anyway? And could Seph overcome Supes's amazing dodges? I think not.