Mountain Laurel Designs makes a silnylon catenary cut poncho tarp, but not cuben. They say that the small sizes on the cuben tarp don't require a catenary ridgeline.
For the past two or three years I've used an Integral Designs poncho-tarp (5x8 silnylon). It worked great as a tarp, I don't think one could expect more from a small tarp in terms of performance. When used right it will stand up to some VERY nasty weather. In terms of aesthetics...the hood is right in the center and makes for a very ugly ridgline. As rain gear it was great, I've worn it while being battered with 35-40mph wind driven rain and it kept me (and my pack) dry. The hard part is staying dry when breaking or setting up camp in such weather...it's just not gonna happen.
I have decided to stop using it as my primary shelter though, and instead only use it as shelter on short, fair weather trips. It will still be used as my rain gear though...I'm sold on ponchos. Instead I plan on using a Tarptent Contrail. I've decided to switch over because you tend to get wet and stay wet (and often get cold) when you set up camp or break it down, as you must take off the pocho and be exposed to the rain in order to pitch it as a tarp. In addition, I used it in combination with a breathable bivy (BMW Vapr bivy) for wind and rain spray protection, but I would always get condensation in the bivy no matter what the weather conditions. That is actually my primary reason for switching from poncho to tarptent, but it is not the fault of the poncho of course.