I'll have to go measure the grade, but I'll bet it is something like 8%.
There's a fine line between too fast and not fast enough. Add to that all the different cars and the different manufacturers, and it gets even trickier. The one critical factor is coupling. If the cars don't couple every time, the whole thing becomes a giant pain. In my limited experiments, I've found Lionel cars to be the most reliable. Imagine that! The coupler scourge of 3 rail O, which stems form Lionel's patent and everyone else having to work around it, is a big part of the problem.
If you look at the videos that Drew posted, the O gauge one is an all MTH layout. I saw it in person back in September of 2013, and actually shot that second video. That layout uses compressed air jets to slow the cars. One other thing to note is that all the tracks are on a constant grade, and never flatten out, so the gravity is "always on".
Here's part 1. It just shows it from a different angle.
Here's my roll test.
