It’s true that the RMR/SRO’s mounting footprint is wider than the width of the Shadow 2 / TSO / TS2 slide. And you’re correct that several companies have figured out how to mil the slide for the wider footprint.
CZC has their plate system.
Several companies, such as Primary Arms, CGW, Scott Millam (formerly with CGW) do it in a way where the mounting screws are longer and thread into the lower portion of the slide where the slide is wider to accommodate the mounting screws, thereby giving it the holding power.
Fire4Effect does it via their own system where they mill out a pocket on either side of the slide and insert threaded blocks that makes the slide wide enough to mount the optic.
Which is the best? That’s up to each owner. I chose to go with F4E’s system, and have not had any problems. I also know people that went the CZC plate route, or the Primary Arms/CGW method, and have not heard of any problems. However personally I like how clean the F4E system is. F4E charges about $300 for their optic mounting system, plus another $300 if you want the slide lightened and refinished.