The OP makes some good points but I think you underestimate the size difference. GSP does have amazing take downs and top control (the best in MMA in both cases probably) and I think he will get Silva down but I don't think he will take him down at will and I think Silva will be able to get back up often enough that a good portion of the fight will be standing. GSP is a hell of a lot better standing than people give him credit for but Silva is still significantly better and add in the size and reach disadvantage and that will make for a tough fight.
Not an unwinnable fight for GSP but I would certainly favour Silva if I were setting the line. It is pretty simple, when a good big guy fights a good little guy, the big guy wins. GSP got to be the big guy against Penn, now he gets to see how that fight goes from the other end. I don't expect it to be as lopsided as GSP-Penn II but I don't expect GSP to win either.