I haven't come to testing either, but you already showed a difference of around 50% extra time. For small render that's no problem, but waiting 10 hours or 15 is a huge difference, so you'd have to check carefully if it's really needed and in what detail/samples. Usually it's only beneficial in foreground I'd say, and mostly on the smoother surfaces. If you have lots of rugged vegetation, it's probably not very noticable. Clouds don't benefit either is my guess.
It would be a great asset if soft shadows could be calculated such that it doesn't take extra time, or hardly! But I'm not a programmer. If I find some time, I'll do some testing.....
