There are pros and cons to it as you said. Before we get to that, a FA QB is not going to have his first year at $800k on a rookie contract, so you are using the last year of that contract to argue that his new one is really $3.2 million less per year than it really is. It is true but not an apples to apples comparison.
My only issue is that if he turns out not to be what we believe he can become we can't cut him for a minimum of 4 years because his cap hit from the guarantee would be astronomical. Heck even after 4 years his dead money (because of the guarantee) would be $40 million. You cut him 3 years from now (referencing your "out") his guarantee would be $60 million right on the cap (I believe).
Not a bad idea per se, but loaded with a lot of variables that could be problematic. Then again he could develop like we all hope then it would be a home run. Who knows, but a potentially scary who knows.