I had a Micro T-1 on it, but used it for something else. Now I am considering putting a RMR that I have on it...anyone done it? Like it?
Which mount might best align to the sights (I guess cowitness or upper 1/3 cowitness?)

My SAN Sig is 553 with the rail

I have an RMR on a low mount I can swap on mine. I use an Aimpoint H1 on a low mount and it cowitnesses perfectly on the Swiss N+ height sights.