WOW this would be fun to read. The thing is though if Superman mastered a GL Ring he would be unstoppable. A GL rings at full potential is much greater than Mjolnir.
He still would probably take down Silver Surfer with Mjolnir. Although if he could send a cosmic/godforce blast at Supes it would probably sting a little. The problem is the combination of Supes Will with the very essence of the GL Ring makes it a very powerful combo.