Slept out in heavy rain and wind a few weeks ago. The only time I felt any rain was when the wind gusts lifted the tarp and the sideways rain splashed a bit.
The problem was two fold. One: I didn't tie the tarp down at a sharp angle. Two: I added about a foot of bungee cord to each tie out that let it rise up in gusts.
If I had tied the tarp down closer to the hammock, I don't think I would have had any problem.
That was my one experience, so far.