>>> I've never used a tarp in the rain. Does it really keep you dry?
Most Poncho/tarps are going to be around 9' X 5' give or take. In wind driven rain/sleet it needs to be staked low to the ground and oriented properly. And the wind direction can/does change. This is where the bivy shines. When your poncho/tarp is pitched in "storm mode" you can normally stay dry with a bivy, but no "head room" as you mentioned. Makes for a long night. Also cooking can be problematic. I have weathered a few nasty storms with poncho/tarps over the years, but a not a common occurance for me. So with experience yes, it does keep you dry, but a bivy is recommended.
Gatwood Cape - I have a Wild Oasis. It is the same shelter as the Gatewood, but no poncho mode. I have used it in rain, light snow and sleet. Have used it when the percipitation did not let up for days... although I hiked during the day. In a bad storm, it has more usuable living space than a conventional poncho/tarp. It will keep you drier, because it is a completely enclosed (and causes more condensation). It is, and does, everything it is advertised to do (if you are 6' tall or less). But it is not my favorite shelter. It has nothing to do with the tent, I just don't like being in a tent period.
With both the poncho/tarp and the Wild Oasis, my main concern is finding a campsite that drains well and will not flood.
So where is this headed for me? I am slowly going back to where I started decades ago. 8' X 10' tarp. I recently bought a SpinnTwinn and just am thrilled with it. 10.1 oz for everything including stakes. It is big enough that in most situations a bivy would not be needed. I just bought a Marmot Essence (6.7 oz), and for me it will probably work as a windhirt too. So potentially I will have all my rain/wind/shelter needs covered for 16.8 oz; for most 3 season hiking.