I like the FastFire. The FFII has had really good reviews and the FFIII is improved.
As a place to start, I just mounted it in the dovetail with a Burris adapter plate - seems very secure, but I haven't shot it yet. I'll just do some slow fire stuff to start with; I'm really eager to see if it makes a difference at 15 yds - thank goodness I've done a bunch of what I call standard eval drills so I have lots to compare to.
I'm not going to be doing many draw and fires with the sight mounted so high, but I will try some rapid fire to see how that works.
The Burris FFIII has a front looking light sensor, so it looks at the light coming from the threat - I think that's the way it should be. The light adjusting auto mode is pretty impressive. I moved from some fluorescent lights and could see the bright mode and swept around to a darkened room at a target in very subdued lighting and I could see the dot dim in small steps as I swept from the light to the target.
I don't see anyway 'top lookers' could do that - they'd still be seeing the brighter area overhead.
IF I REALLY like the RDS, I'll likely buy a Lone Wolf slide and machine it for the FastFire and put the suppressor sights in as well. The mounting posts, that insures the correct mounting position of the sight, is the only thing that makes the machining a bit more challenging on a manual milling machine, but I can do it and have done such things before, it just takes a bit more human intervention than a CNC machine would.
I'll keep you posted. Gonna be a fun and interesting project.