You all seem to be forgetting that once Seifer started working for Edea/Ultemacia that he stops using the GFs. The GFs allow SeeD to fight the Sorceress' on equal terms and the game goes out of its way to show the Sorceress' are forces to be reckoned with. Its why he gets weaker as the game progresses cause he's normal while Squall and company get stronger GFs. Yes on an emotional level and psychological level I would agree Squall is stronger,but Seifer continues to fight GF powered humans and GFs all by his lonesome. Granted he never wins but the fact he survived and somewhat held his own shows his strength.
Just my thoughts though...