I recently bought the 45mm TS-E. I wanted something longer than my 24mm TS-E but felt the 90mm was too long. Then I read that the 45 will work with extenders. So I bought the 45 and used it with my 1.4 extender yesterday. It worked but I was surprised when I discovered that 1 stop was not lost with the 1.4. I could open up to f/2.8. At least that's what the camera is telling me as well as the EXIF.
The EXIF data also says the images were shot at 45mm when there was clearly a longer focal length with the 1.4 attached.
So that leads me to believe that the camera is not getting the data when the 1.4 is attached (it becomes a 63mm f/4 lens).
Not a big deal...the image quality is fine...just curious.