You could always get a magnifier on a swing out mount for your RDS.
For how far out something would be effective.. that kind of depends on you. Are your eyes bad? How accurate are you normally? My 3X is good but not special to at least 225. (max range at one of my ranges) I have a 6x24 Varmint rig for plinking fun that I never really run up high, so I don't know why I have it... but the higher I set it, the better I can see, but somewhat shoot worse when factoring in how I can see.