My Postwar 1950 773 pulls a dozen postwar cars up a 5% incline. On a level surface I think you could double that. Add in modern cars with great trucks and balance and you might be really surprised at what it would pull.
Remember the curve diameter is the real limiting factor.
My 773 is my favorite postwar engine. My Lionel SD70ACe is my favorite modern engine and it will pull 27 cars up and over the 5% incline. (Careful balance on the weight distribution - heavy in front and light in the rear. Ha Ha...)