That is one theory many have. Saito is a very mysterious character and I have heard everything from he was hired by Cobb's in-law father or that Saito is really a manifestation of Cobb and the whole movie is a dream. The first time I saw it I did think some type of Inception was done on Cobb but on second viewing a believe the movie was presented as is.

That scene that seemed all magical for Cobb the first time I saw it was simply two guys remembering a conversation. The lines they said to each other were simply from a conversation they had earlier in the film. Both of them had forgotten why they were there, which is the danger of limbo. When they met they were able to remember their mission and remember that they were in a dream. Both Cobb and Saito were saved in that scene, not just Cobb. The reason everyone went to Cobb's limbo is that limbo is unconstructed dream space but if someone had been there before it would use that image, Cobb is the only person to have been there so Limbo looks like his dream. The look on the faces were simply a "holy shit we got out of there" look both Cobb and Saito had that look cause they saved each other from Limbo.

Now the story of Inception is definitely about Cobb learning to let go of his guilt. I believe that has a lot more to do with Ariadne. She mysteriously knows and cares a lot about Cobbs past. I believe Cane's character gave her a side mission, to make Cobb confront his demons. He was able to do that in this mission.