I agree that 3 metres of ascent in 10 metres along the slope, is about 17.4 degrees. That is too much for a Briggs. If Honda says their engines will tolerate 20 degrees, it sounds as if you can just make it with their engines - provided you watch the oil level in the sump closely, of course.
Personally I'd rather not run an engine for substantial periods at close to its maximum rated slope, I'd rather use a 2 stroke, despite disliking 2 strokes.
