That sounds about right based on what I have seen in the few heavy trailers I have towed.
But my point is that with a proportional controller, like a Prodigy, if your EB is on and slowing the load if you touch the brakes enough to engage the Prodigy it will brake the trailer in proportion to the braking energy of the exhaust brake, but a Max Brake wouldn't do anything that way.
There are some winding roads around here where the EB holds the speed perfectly, but you need to slow a little on the corners, with my Prodigy I just have to touch the brakes and the trailer brakes come on and slow me the required amount, if I need more slowing then I press the service brakes and get braking from the truck.