I got the DA*300/4, the Kenko pz-
af 1.5x SHQ and a Pentax A 2X-S.
The Kenko doesn't autofocus well with my K10D.
SDM works, but only if I prefocus manually close to the object it will lock focus. I have read that with the K20 and with the K7 it works better. The image quality is not bad, but a little soft. Definitly it could be better, but you still gain detail in comparisn to the 300 without TC at 100% view.
The Pentax A 2X-S has great optical quality, clearly better then the Kenko. Excellent details at 100% view. Evidently no autofocus, behaves like an A-lens (anyway, with f=8
AF won't work very well). Focus trap works fine (I assigned
AF to the
AF-button only; if I want focus trap I push
AF and shutter release buttons at a time, and move the focussing ring.
As effective scale increase I have measured 1,52x with the Kenko, and 2,0 with the Pentax.
Best regards - Klaus