Mpix black and white is pretty crummy, in my opinion. I like their color prints, though. enough so that I switched my Epson 1400 to MIS inks.
I should say that, after a few days of grappling with the MIS inkset and running through a bunch of ink, I am now making STUNNING prints that use VERY little ink with the help of Quadtone RIP. The difference between printing with QTR and out of Lightroom is phenomenal. Deep blacks without excess ink, black-only option, super fine detail.
I'm still new at it, but it seems to me that, for the time being, getting a good workflow going at home with B&W, and sending out for color, is the way to go.
Also, I'm curious how you'll hang a half-color show...seems like it might be an interesting challenge.