Different engines can pull different amounts -- even on level track, due to the weight of the engine, and the traction of the wheels on the rails. Traction tires that came on older Kato/Atlas units made a difference; I have an RS11 that had them, and it pulls about 1/3 more than the GP9 with identical drive but no traction tires.
I got about the same results out of my Kato F7's....but the Athearn F59PHI's will pull about double that, though they do slip/slide a bit when first starting. (Better than the wheels not doing anything and buring up the motors...)