Eric, thanks for posting those URLs.
>are the underpad's worth it in cold weather, or would a cheap closed cell foam pad work just as well.
For temperatures below freezing the Hennessy under-pad/under-cover system would work fine with your sleeping bag. The advantages over a typical closed-cell sleeping pad are that it covers more bottom area (most sleeping pads are 24" wide so you will need to glue or tape two to make a wider pad), and it's a whole lot easier to deal with slung insulation than in-hammock insulation. However, a Gossamer Gear ThinLight Evazote sleeping pad (Wide-Long if available) slung in a Jacks R Better Weather Shield bottom cover (at present, only their top cover is out of stock) is much cheaper, lighter and almost as warm as the Hennessy under-pad/under-cover system. The JRB Weather Shield has the advantage of being able to sling a quilt as well as a sleeping pad (see below).
>lows at high elevations run around -5-0 at the very least.
For temps of -5F to 0F, I don't think a Hennessy under-pad/under-cover or a sleeping pad-based system will cut it. I consider that system comfortable down to +25F with a quilt and insulated clothing. I'm just guesstimating, but compressed sleeping bag is probably a bit warmer than compressed Patagonia Micropuff, so you might be able to go as much as 10F to 15F lower with a 0F synthetic sleeping bag; that might keep you warm into the teens. YMMV, however. If you can test your hammock configuration where you can bail home if you freeze, it might be worth trying an under-pad system.
For -5F to 0F temps I'm comfortable with a Jacks R Better Nest quilt slung underneath the hammock--a full 2" of down loft. Since the Hennessy under-pad is only 3/4" of open-cell foam (generously overlooking the 1/2" dimples), there is clearly quite a difference in the amount of insulation I need. If you have a down or synthetic comforter or quilt that is about 2" thick, then you could try slinging that underneath (easy with the JRB Weather Shield bottom cover). If it works for you, then you could replace it with a lighter quilt.
> also what about the over covers?
I think the Hennessy over-cover does help a bit, especially if you don't have a heavy enough top-quilt. I've used mine several times, and there is always frost condensed on the over-cover; it doesn't work like a double-wall tent. If there is a cold breeze blowing under your tarp then the over-cover will really make a difference. However, in winter I usually stake my tarp down close to the mesh, so that does about the same thing. I do bring the over-cover when it's below the teens.
> I thought I would get the proper gear now for next year.
The proper gear for hammock-hanging is harder to determine than for ground sleeping. The sleeping bag/tent environment is fairly well understood, and the top-quilt in a hammock is similar. But bottom insulation seems to be much more variable. As they say, YMMV. Considering the consequences of temps in the teens, let alone below 0F, I suggest testing in a safe environment first. There's nothing I hate more than waking up shivering and then stopping, wondering as I fall back asleep if I just warmed up a bit or if I'm going into severe hypothermia.