space-time migration and kamui seem to be quite inter-related to me. From what I can guess, Kamui is just a lesser version of space-time migration, similar to how Fire Style: Dragon Fire Technique Fire Style: Great Dragon Fire Technique and the Fire Style: Grand Dragon Fire Technique Fire Style: Dragon Flame Release Song.
Since they lead to the same dimension, then it may even be safe to say that if Kakashi were to look into a mirror and use kamui, he might end up warping himself into the pocket dimension Tobi is in. I'm guessing that Tobi has just been capable of training it to a much higher extent, since the technique clearly wouldn't take near as much out of him as it does Kakashi since Tobi is an actual Uchiha. Since he hasn't gone blind, I also presume he has an eternal mangekyou sharingan in dat eye, or has been "borrowing" all of those Uchiha eyes whenever one of his goes blind...
I dunno, just my thoughts.