Originally posted by BigMackCam The OP lives in the same region of the UK as me where, for a large part of the year, we don't often benefit from clear, sunny skies.
Masterful understatement Mike!
I have the Kenko 1.5x TC with pz contacts. AF works using this with the PLM in ideal conditions, but even in this part of the world conditions for photographing wildlife aren't always ideal. When I have used it I have wondered whether I gained anything compared to just the PLM and cropping.
The comparison by @Swanlefitte is interesting. See also comparisons posted by @Normhead between TC and cropping. My takeaway from these would be that a TC
can capture more detail with a fast sharp lens in certain situations, but the returns diminish when the lens is less fast or less sharp. Looking at Chris's test I would say that there is some gain in detail from the TC + PLM at 300mm v 300mm and cropping, but it is pretty marginal. There was more gain when he backed off to 210mm, because the PLM has more resolution there. But I would expect that a good 300mm prime would have done better with or without the TC.
Given the cost of the DA converter, personally I would put the money towards a good 300mm prime instead. I have the FA*300 f4.5 and it's a very sharp lens - around $A800 here, compared with around $A600 for the TC. Or consider selling the Sigma and getting the DFA 150-450.