Here's my thoughts on custom Glock slides.
First, a question. OP, what is it (other than an RMR cut) that you hope to achieve with a new slide?
Do you:
A) Hope to improve your pistol shooting abilities by getting a custom slide? (again not considering the RMR)
or
B) Just want something that's going to give you and the guys at the range a woody?
In my humble, but forced oppinion, the buck stops at Black Box if you want to do a custom Glock with RMR. The guys at BBC do fantastic work, and I would agree that most of their offerings will offer you some improvement over your factory Glock.
That being said, spending 1600 dollars on an agency arms Glock slide seems absolutely preposterous to me. For the same price, you can buy a brand new RMR, custom work at BBC, AND a case of ammo and depending on your abilities you'd likely still be better off with the latter. Over priced Glock slides like Agency, or Zev don't offer anything better than a custom job will.
Unless of course your answer was B in which case, do whatever gets your #### up!