As with any custom gunsmithing, you're throwing the S&W warranty out the window, and once it's done it can't be undone. If the shop screws it up, you're, well, screwed. Milling the slide is also going to cost $200+ with refinishing.
In my mind, it makes more sense to sell the gun you have, and buy a factory gun with the optic cut. It may even cost less than having the slide milled in the current market depending on what you can get the new gun for.
Of course, if they didn't make the model you want with the optic cut, you're kind of stuck if that's the gun you want.