I'd say the absolute maximum would be about 4 degrees, and with O31 track, you'll need a few straight sections in the mix to get that with 31" diameter curves. Obviously, you'll need even more for O27.
You need 144 inches of run to go up 6" for a 4% grade. When you're going on curves, that places an additional load, so the effective slope is even more, probably by several percent.
In my testing for my helix, I'm running into problems with the inner track slope with O31 curves and six straight sections for each 6" rise. Only shorter trains can make the trip, maybe 10-12 cars max. I'm currently rethinking what I want to do there.