Do the SM58 and SM57 mics have the same frequency response curves? Is an SM58 simply an SM57 with an integral pop filter/windscreen?
The SM58 and SM57 do use the same cartridge. The grill, though, does effect the frequency response to some extent. At a distance, the difference is probably not noticable. But at extremely close distances, the difference is noticeable. Due to the grill, a person's mouth can get closer to the diaphragm of an SM57, thus causing more proximity effect and increased bass when compared to the SM58.
Note that the cartridges are not interchangable because the mounting scheme is different for the two microphones.