It's not accuracy, its POI shift, ie: a changing zero. If there's pressure on the handguard it will shift the POI. You're basically doing the exact opposite of a freefloat bbl. Instead of handguard pressure affecting POI due to moving the bbl without moving the optic, you're affecting the POI by moving the optic without moving the bbl.
In Green Eyes and Black Rifles Kyle Lamb states that depending on the amount of pressure applied one can move the POI 4" at 100 yards with a non free floated barrel. This is not an insignificant amount, especially at 200 and 300 meters, which is entirely within the practical capabilities of a RDS. This POI shift should also occur with a freefloating bbl and a handguard mounted optic. You might not be grouping, but shifting 8-12" off point of aim can put you seriously off a human size target.
That said, the closer to the receiver, theoretically less shift should occur.
What aimpoint are you using? If it's a standard PRO/M2/M3 tube style there are several manufactuers that make cantilevered mounts. If you have an M4, GG&G make one. There aren't any I'm aware of for the Micro series.