As a mic designer (I designed the Nuetell V666, V2001: A Stage Oddity, and V911), I just wanted to correct you guys on a few points.
First of all, removing the ball windscreen from the end of the SM58 does not complete its metamorphasis into a 57. What this does is loosen the magnetic field around the torridal transformer beneath the capsule, allowing it the "flow freely" into the air, as opposed to being "constrained" in a ball by the metal windscreen. This results in a smoother frequency response.
After you remove the ball, however, you need to put some sort of electrically-conducting grease over the capsule (I prefer graphite powder mixed with silicone), then put the mic in the oven (top rack works best) and bake for a good 2-3 hours at 375 degrees. I kid you not. This will bind the magnetic abberations introduced by the oven's elements (make sure you use a conventional oven, not convection) to the capsule in a way similar to that done in the manufacture of a SM57. Yes, the capsule is the same in both mics, but the after-process is not. Usually Mexican immigrants do the baking process cheaply, taking little care for proper kitchen safety and smelling tests; this is why older SM57's (all-American) sound more appetizing than newer ones.
As for the "graphs" posted previously by another member, I don't know what that is all about, it must be some stupid joke. I mean, a smiley face for a frequency response? Get real buddy.
BTW, if you accidentally wreck your 58 by setting the oven temp too high, just smear some more of that grease on it... all over the mic. Then bring it into bed with your girlfriend and all of a sudden this "general purpose" do-it-all mic has found its favourite application

.
Good luck fellas.