For what is worth…… I have just ordered the 20D 1.1oz silpoly PU4000 for my next tarp-poncho project. Although 30% heavier than the 15D membrane silpoly PU4000, the total weight penalty is not too big since you use very little material. (in my case 3+ yards). I assume it will be used more on tarp mode than poncho mode, so for me is good to have slightly better tear strength.
I made a hoodless tarp-poncho prototype of silnylon to see if the tarp-poncho would work for me. (I never used a tarp this small) I really like it but the lower aged HH and the fact that silnylon gets heavier when wet made me give silpoly a try.
I really like the coverage it provides in Half-Pyramid pitch with a removable curtain in front. My plan is to have two curtains. One for rain (waterproof fabric) and one for bug season (Noseeum mesh).
Edit: Pictures added of Hoodless tarp-poncho prototype w/ Removable Curtain


It was a very windy day, great for testing a tarp :-).