This doesn't answer your question, but your problem before the question.
If your brake body is moving about, the problem isn't the existence of the shim (which is used by all brakes when they upsize) but either:
the bolts holding the shim/adapter to the bicycle or the brake to the adapter were not properly torqued and loctited.
the shim was poorly made and has a defect that is causing it to move
Note that brakes are supposed to move when the bolts are slightly loose so that you can properly center the pads against the rotors. Then you clamp them down to the proper torque tolerances.
Since most people want to go up in rotor size to have more effective braking, you'd be bucking the trend in going down. Are you 100% confident in the mechanical skills of your LBS?