Interesting that you had problems with the sensor. I never did, or at least I don't think I did. With the three Mk3s I used, what I was seeing on Live View was exactly what I was getting. Of course, live view can only see what the sensor is seeing but my point is that it was perfectly aligned with the grid.
On the other hand, two of the bodies had alignment problems, while all three (including the one I now own) had/have problems with clipping across the bottom of the viewfinder.
I guess you could argue that the viewfinder frame is correctly installed and that the sensor is not installed to match it! But, my money is on the viewfinder frame not being the 100% viewable area that it should be.
Bangs for my buck, the 5D is still the best camera I have ever owned and that includes all my film cameras, with the possible exception of my Sinar P.