I think with using a very ligthweight fabric problems will only arise when the stitching is bad.
Its very important to keep an adequate distance between the holes. Check the other factory-made seams and duplicate the distances. when reinforcedment stitches are done, they shouldnt be zi-zagged all over the place, but placed regularly.
I own a ultra leightweight Silnylon Poncho from Sea-to-Summit and will convert it the same way without worrying about its strenght.