Jackster,

The beauty of CNC machining is that you get 200 sleeves and 200 pistons (and crankshafts, and...and...) and you take ANY one of each. They all fit their mated parts and all feel the same when spun around.
They will also be within 100 RPM, as far as their performance is concerned.

There are no "bad apples". The entire lot is either all good, or all bad. And a minimal measure of QC makes sure this doesn't happen, or the goods don't leave the factory.

The fit between the parts is such, that they actually do need a minimum of break-in time, to achieve the correct fit.

CNC is used to machine valves and their seats, for full size engines.
After that, you don't need to grind them with silicone carbide paste, to make them seal. They seal like ground valves and seats, immediately after being manufactured, without the added hassle.

Todays CNC manufactured engines are very close to that, but in the case of cast iron rings, or pistons, the heat treating - annealing, work is up to the modeler. By doing it properly he, or she, will determine the lifespan and the life quality of the engine.

They have not yet found a way for CNC to replace the annealing, but who knows? Maybe someday.