The rainshower is very disappointing unless there is adequate volume and water pressure.

Before you destroy the shower head, start by cleaning any debris from the shower control valve. Take off the lever handle, remove the valve cartrige, and clean out the screen inside the mixing valve. This is the most common cause of low flow issues with a thermostatic shower control. I'll bet the screen is totally gunked up, especially if you've never cleaned it. I recommend at least one time per year, or when the water flow drops in the shower.

After you are sure the valve is free of debris, and if the flow is still inadequate:

If the regulator is plastic, I drill out the center with a 1/8" or 5/32" bit. That's the best way to maintain all the proper functions and improve the volume of water flow.