I understand the Torque reduces. and slower Cutting speeds are needed for harder stones. so faster cutting speed is not a good idea .
there are many types of stone grinding spindles 3KW Grinding spindles at 5000 RPM are available which means Oceans of clean cold water any loss of water means a new Grinding bit .
Though I have been quite surprised as of looking at the Technology of some of these spindles many are designed grind and polish through Granite worktops with high KW spindles. so wouldn't have a problem with Sandstone or limestones. such as Portland, Purbeck and soft stones like Ham and Bathstone.