The max PSI stamped on the sidewall is the rated maximum pressure the tire can handle at load. It is NOT a recommendation to use that pressure. I would imagine at 50psi you will get some uneven wear of the tire.
The door sticker on my car suggests 30psi. This is based on the tire size and load rating, and a different brand of tire that meets the same size/load rating would be similarly inflated. A GS-D3 doesn't need 20psi more than a GS-C just because of the rated sidewall markings. I would think a pressure in the 30-40psi range would be a lot better in terms of traction and wear.
Interestingly, the door sticker says 30psi, but I just noticed the service manual says 35psi for coupes.