Hi there,
I have an MTH J in 2 rail that pulls 12+ scale cars up a 3 percent grade with no traction tires, no problem. I did add weight to the engine. There is a 56" radius curve on the grade. Just something for consideration in the mix.
For freight trains, I don't add weight. I just add engines or helpers if necessary. It's more fun this way.
Taking traction tires off an engine and leaving a groove would be a ridiculously flawed test.
Importers adding weight might add considerably to shipping cost and damage in shipping which might be a consideration.