Listen to Wilkee---the Behri XM8500 does good SM58 duty (and 57 duty if you take off the ball) and is ridiculously priced---well worth the $3 price difference.
I'd say the XM8500 is pretty close to the SM58, but I actually find my XM2000 better on most voices, and even good on a few - even if it's not supposed to be a pure vocal microphone or a SM58-clone.
Probably there's enough variations in the production for these mics to get all sorts of weird results.
I rarely use any of these mics for keepers though...