I have a Primary Arms RDS with about a one inch riser. It's quite comfortable to shoot with that. As 124gr9mm notes, you really need to get into an uncomfortably low position to use the iron sights (plus, suffer from cheek smack), so I assume the same would be true with a lower RDS.
Makes a lot of sense. I know that purists like a co-witness in case the RDS stops working, but I have found it to be extremely uncomfortable on this gun, so I guess I'll just leave it as is.
Thanks for the input.