Malcolm......the disappointing (and expensive) foray for me was to have the DA*60-250 and finally find a FA 1.7 T/C. YIPPEE!!!!Your 100-300mm interests me greatly,I had always aspired to the pentax 60-250mm to replace my smc100-300mm(outresolved and generaly sub par on the k5) and have seen some cracking shots from it,however the lack of a 1.4 teleconvertor(i cant believe Pentax dont think its worth making one)and the old 1.7 adapter being like hens teeth to find! have me looking elsewhere!
EXCEPT the combo didn't work!!!
So that's how I ended up with the Sigma 100-300.....and am happier for that.
R