I sent mine to Primary Machine. They are Louisiana boys in Morgan City. They offer black nitride refinishing, which is as close to an original Glock finish as you can get. Everything is cut on quality Haas equipment and they are perfectionists. ATEi is good to go as well, but I have zero issues sending it to Primary Machine, because I know they do good work.
Turnaround is 2-4 weeks, depending on what you want. Might be longer depending on how much work they have. Optics cuts start at $150ish if I recall. They can also supply suppressor height sights if you need them and they will install them at no extra charge. You can ship the entire slide and they will disassemble and reassemble at no charge as well.
I also sent a Glock slide to Primary for a Deltapoint Pro cut. They specifically asked that I ship the sight with the slide, so they could ensure the closest possible fit. It was close. Very close.
And the slide refinish was top notch. No complaints here.
If you don't mind using stock M&P suppressor height sites I just picked up a M&P C.O.R.E slide that comes with the optics cutout and co-witness sights. It also comes with a ported slide and barrel to go with it for about ~$370 which is about what it would cost for most places to just do the slide cut.
From the comments, it seems like it will work on both gen 1 & 2 pistols. From what I read if you use it on the 2.o you just won't be able to slingshot the slide forward and will have to always use the slide stop to move the slide forward after a reload. Although they say you can replace the 2.0 slide stop with a 1.0 version and it will slingshot but I always use the slidestop to move it forward so it won't bother me. Probably try it out on the 1.0 first to see if I like it as the 2.0 is going to be my main USPSA gun and this kit will put me in unlimited with the STI guys.