During the Cryo treatment the grain structure closes much like taking your other hand and squeezing your fingers together, thus making a better bearing surface with less drag and friction and therefore improved performance. Normally you will experience a slight increase in Rockwell of a point or two, not much of a change, but still improvement in the right direction. The big improvement is stress relief and stabilization. Engine parts, transmission parts , or any non cryogenic treated parts you are thinking about are full of stress and just waiting to distort when it reaches racing temperatures. All machined parts can distort if they have not been stress relieved.
Pistons can change from round to oval due to stress and temperature.
When this happens the piston drags the cylinder wall which takes horsepower away, creates heat, and wears the cylinder walls. It then requires the engine builder to rebore the cylinders by 2 to 3 thousandths. The end result of Cryogenics treatment of your engine is improved performance with far less wear and less breakage. All parts that wear, break, or distort. Sheet metal, electrical parts, water pumps, oil pumps, or fuel pumps should not be treated.