I agree, the convertible is just a cool factor, I suspect the real car will have a roof similar to the current CTS/Sixteen. GM is just testing the waters with the overall design of the front, rear and sides. I suspect the front end of any such car would look more like the Converj. However there was a time when GM made wonderful full size( and I mean much larger than the DTS) convertibles that were amazing machines. I fondly remember them as a kid riding around in the folded up part and staring up at the stars.
BTW, the Ciel has a length that is just about the same size as the current DTS, S-class, 7 series. So its going to be reality in some form close to this.