Hello Christopher,
This is a rather tough question to answer because so many things come into the calculation of traction.
For example: 20 loaded cars on a 2% grade is fairly close to the prototypical limit for a 6000hp locomotive! 20 cars x 130 (gross) tons = 2600 tons + 200 tons (locomotive) = 2800 total tons. 6000 divided by 2800 tons = 2.14 hp per ton. For the sake of argument, assume a railroad uses 1 hp per ton for each 1% of grade. I think AC traction can get lower, somewhere in the .7 or .8 hp per ton. Interesting, all our models are DC -ha, ha.
So, if your cars are prototype weight and you locomotive has protyotypical traction, you are maxing out, just like the prototype. If the locomotive is either: 1) a little slippery; 2) not as heavy as prototype; or 3) the cars are heavier than prototype, then the traction will decrease. Also, each degree of curvature incrases traction demand, and our model curves are VERY sharp, requiring a good bit of the locomotive's traction. Kato may be a little slippery, as the Bachmann C40-8W pulls more than the nearly equivalent Kato C44-8W in a test I remember seeing in a hobby magazine.
Also, the condition of the track and wheels affects traction. Clean, dry wheels and track = maximum pulling. Gummy or oily track and/or wheels = extra drag (gummy) or slipping (oily).
For practical application, a quick test bed is the best answer. I suggest taking an 8-foot long straight, connect one end to a 180 degree curve, and connect the other end of the curve to another 8-foot long straight. Since 8 feet is 96 inches, for a two percent test, the starting end of the first 8 foot section is level with the floor, the opposite end is 2 inches higher, if your mainline curve is 19" radius, you would add 1-1/4" of height for the length of the curve, then add another 2 inches to the far end of the second straight section. Your railroad should climb a total of 5-1/4" for this distance (96"+60"+96")For 3%, just add 50% more to each height.
To measure length of a curve, use the old geometry equation:
Circumference = "2 x 3.1416 x radius" (this is a 360 degree or full circle curve)
for a 180 degree or half circle curve, use "3.1416 x radius = length"
for a 90 degree or quarter curve curve, use "1.57 x radius = length"
This is the best way to test total traction required on elevation and curves. I chose 8 feet because that is a standard lumber dimension, and if you want 20 cars, that is pretty close to 8 feet or longer, depending on the cars you are pulling.
I have built my model with 2% grades, but since my trains will double- or triple-head like the prototype, I won't have any traction worries with my 8- to 12-foot long trains.
In a nutshell, I think you got all there was to get out of your Kato!
Happy modeling!