In regards to Grey Hulk beating him, and even before Krypton Returns Kon had some impressive feats like blowing a giant hole into the earth's molten core and flying through the earth's crust to the other side of the planet.
As well as destroying the Star Chamber which even H'el thought was impossible in the H'el on Earth storyline.
There's also the fact Kon could fight Kara in Superboy #6 and she couldn't do any harm to him because of his shields. Then he caught one of her punches and incapacitated her with a blast of his psionics.
Also Kon has some measure of superspeed through telekinetic hyper-acceleration which we see in Superboy #3 and later on as in Superboy #20 IIRC.
Kon also has the ability to transmute people but I remember the Hulk being immune to that like with the Grey Gargoyle.
After Kon master his psionics when Jon Kent gives Kon his memories in Teen Titans Annual #2, which Krypton Returns is set afterwards he is also powerful enough to manhandle the Jon. Able to throw Jon into the moon, and fly through it himself.
In the final part of Krypton Returns Kon is able to throw the entire city of Kandor into space with telekinesis, and defeat the Eradicator earlier on. Not to mention H'el himself but H'el was also fighting Supergirl in a different time-line when that happened.
Getting my Superboy's mixed up. If Superboy is as amped as I think he currently is, he could probably make it to WBH. He is the most powerful meta in DCNU right now (Earth based Meta).