Some have a limited budget. Just by going Magneride you are adding $6,745 to the price of the GT - that's a lot more than 2% - closer to 15%.
Maybe check your math before you post?
3.15:1 will be the better for daily driving and fuel economy. The 3:55:1 will result in faster acceleration (if you can get the power down) and slightly less fuel economy.
Other than the slightly odd front end, you can't go wrong with the M4 -- especially the M4 Competition. They are already amazing and with a tune they really wake up.
I'm so glad I don't have to run a front plate. Unfortunately my dealer pulled my C/S from a lot with a State that requires one and they drilled the bumper. Had to get them filled.