Ninja -- NEX-5 & Kowa Super-Prominar-16 Projection Lens. Click for larger.
So I have replaced the c-mount to NEX adapter on the Vivitar 2X macro teleconverter focusing helicoid with a 49mm NEX reversing ring. The opening is now much larger than the c-mount's 25mm diameter. The first lens that benefits is the Kowa Super-Prominar-16 projection lens. This lens was not able to focus to infinity before because the c-mount was restricting the lens from going further into the lens mount (camera body). Now that the mount is larger, infinity focus is possible.
It turns out that I like this lens a lot. Sure it vignettes a bit, and there is purple fringing, but it's very sharp and the depth of field is so thin, even when shot from quite a far distance. Perfect for low light or when thin depth of field is needed.