Here's a few photos to clear up some coverage issues related to wind with a diamond shape tarp and the Speer WinterTarp. The WinterTarp shown is a prototype I made with white silnylon when I was finalizing the catenary darts to help keep it taut. I utilize shock cords on the side tie outs to help keep everything taut overnight. These photos are from Oct 2007 at Hawk Mountain Shelter on the Georgia AT when Joe and I did a backpacking trip.
With the WinterTarp you can block the wind pretty well. I choose a setup with the ends not totally closed off so I could get some ventilation, have a little more room, and have an easier entrance/exit than I would have had if I closed it all the way off. I have had to get up in the middle of the night and close one end off when the wind came at me from a direction I wasn't expecting but I don't close it all the way off unless I really need to because of space and ventilation/condensation considerations. There is not a whole lot you can do with the diamond shaped tarp but hope the wind only comes at you broadside.