I have done this modification to an SM57, and was very happy with it. The output is MUCH lower, as to be expected, but the sound is wonderful. I have gotten no noise or hiss from this mod. It really does sound a lot closer to my SM7 now. After removing the transformer, I added a Rode D-Power to the inside of the mic. This required boring some of the metal inside the mic to get the D-Power to fit. This is not a mod for the faint of heart, as you can easily destroy your '57. After adding the D-Power, the mic had a much higher handling noise and some nasty resonances due to removing much of the internal structure and the glue holding the transformer in. To resolve this, I filled the mic with hot glue. This brought the handling noise back down to original levels and completely removed the new resonances. One thing now though, there is some hiss coming out of the D-Power. It is not usable on low level sources, but it works great on rock vocals, keys, drums, and percussion. I haven't tried it on guitar or bass yet. It's definately worth the $90 to try removing the transformer from your '57. You will get a new mic that sounds very different from the rest of your '57's. You will, however, need a pre-amp with a lot of clean gain to get usuable signal from this mod.
Cheers,
Zach