Are girdles generally added for big rpm or HP or torque

I agree. However I have heard first hand how bad the factory balance can be and I have been in the very car that had the very engine that didn’t vibrate in he car ran. It was a smooth ride. The cars owner must have had a discussion with the builder on this topic and the engine builder proved to him how bad it was but spinning the OE rotating assembly.

The builder claimed the worst he ever had was 7 grams. Which otherwise, I would have never believed in.

If the assembly can be finished to a 100th a grams poop, then go for it.