Slade overpowered the ring temporarily. In Identity Crisis, Slade's willpower and Kyle's willpower were evenly matched. Kyle won in the end. The battle between him and Slade didn't make much sense though. After Slade handed Green Arrow and Hawkman their asses, Green Lantern rushes in and hits him with one fist. WTF!? GL could have just blew him away with a plasma cannon. GL NEVER engages in hand to hand combat. It was a plot hole to say the least.
As ION, he had God-like powers. He once dispatched an entire alien fleet with millions of spacecrafts that was light-years away from the JLA watchtower in a picosecond. That's how powerful he was.
Green Lantern can do ANYTHING with his ring. There's no limit to his power except his imagination. He once contained a supernova the size of 100 Jupiters with his ring. He can do ANYTHING.