i think the whole discussion is really based upon whether you are close to filling the viewfinder with the image or not.
i used the AF adaptor for years as noted above with the K300/4, and in combination with the AF500FTZ on the *istD (note that the *istD supports TTL flash) i could get incredibly close with this combo to very small birds like chickadees etc, (less than 100mm in length) and get great images, with only a small amount of either fringing or lateral CA. see below 90% of frame crop for composition
i have done longer shots with things like herons etc, but where this combo breaks down is when the image is very far away, (consider this as equivelent to the lens on the infinity stop and the AFA focusing. in this case, it is questionabnle whether the AFA does anything to add to the quality or not.
As norm suggests, there is a range where it helps, i would suggest perhaps more than the 20-30 feet, perhaps our to 100 feet, depending on size of subject etc, but clearly no further. the subject is just too small at distances further than this to resolve any additional detail.
note that there is also some portion of luck, as i managed once to stack a sigma 2x on top of a 1.4x and achieve AF and a good shot of a whooping crane, when using a sigma APO 70-200/2.8 EX but that was a one in a million shot hand held from a boat
see below 10% of the frame from the center on a K10