The problem is that as it stands the 2.39:1 caps look small in comparison to the pages that use 600px for 16:9. I think we should either increase one or decrease the other. Also a 700px 2.39:1 image is still vertically smaller than a 600px 16:9 image, so it is not making the page comparably "longer".
Just for reference, can you link to a 2.39:1 screencap?