Is the rain fly that comes with the hennessy expedition suitable for heavy rain or should I upgrade to the hex rain fly.

Well, that's a good question.

Personally, I have the stock fly, and have been through some good storms. I would be lying if I said I would be happy with the coverage, but it mostly has worked out.

I just bought an underquilt cover from JacksrBetter, made from a Driducks poncho that I think will be enough for the side rain where I'm a bit low on the fly.

One thing - if you tie up the fly correctly, and aren't pushing the weight limit of the hammock (like me), you can get a pretty nice coverage - see Derek's page

One more item - you can always tie the fly separately from the suspension, and keep it where you want it. See Derek's video of a continuous loop.

I have no personal experience with the Hennesey tarp.

The Noah's 12' tarp has served me well through some serious coastal storms (60/70 knot winds with higher gusts). Never a drop of water on my hammock or insulation, Plenty of coverage. On the down side, it is heavy.