A lens I would like to see built.

I sure would like that TC to appear sometime soon.... if they can get it to work.

From the trials I was performing with a Kenko 1.5x Pz-AF at the weekend, the problem wasn't really with SDM, but with the focusing method used.  When in Live View, switched to CD-AF, the combo of TC + DA*300mm snapped to focus every time.  Back to "normal" PD-AF and it wouldn't lock without a lot of jiggling of the AF button.  Perhaps that's why we have not seen it released yet?

Agree though that a DA*400mm would make my life complete, at an affordable price of course.  The 560mm Tele, while nice, is just so massively expensive and out of the reach of many of us mere mortals.