Well, I just ran a test on my office layout. I ran all but one of my locomotives pulling a short ore train (two hoppers plus a bobber caboose) up a 10% incline over turns as sharp as 9.75 inches, and they ran fine. It barely phased my Bachmann 2-8-0 and Life-Like SW-9/1200.
Of course, I'm not planning on such an extreme grade on the 2x4 layout...