Hi, I have used a modified one and thought it sounded interestingly different, but wasn't a revelation. It's not like a 57 is a rare or expensive mic, so I would say go for it. I am not a big SM57 fan, but understand that a lot of people insist on using one and don't mind quite so much when they're using two mics (like a 57 and 67 or a 57 and a 121 on amps, or a 57 and some other mic on snare, etc).
I'm thinking of modding a transformer into my SM7b. That's a whole other thing to think about, though...