Im running 17 on my 95 540i with the suspension mods. The tires say 51 psi max pressure and that would be at full ride load. I have been running the air pressure 38 or 40 psi. The ride changes quite a bit to me between those two pressures. I think 40 is where I will be keeping it as it rides pretty darn good except for the worst roads, or the train tracks out here at least. I have been watching for tire wear and haven't seen anything as of yet. The tires only have about 4K miles on them but I would think if there was an issue it would be visible by now.
Johan