Let me tell you a little story. I bought the sam TC from KEH a couple of years ago, for about the same price. I have not used it much because I did not like the results I was getting. I then bought the Pentax F1.7 AF TC. I sold that a few months ago. A couple of months back I picked up a Sigma 100-300 f4.0 EX DG with the matching Sigma 1.4x TC. This combo seemed OK.
Last week, I was out at the beach with this combo. I dropped the TC on the beach
. Well to make a long story short, the TC won't AF, so off it goes to Sigma to be cleaned and repaired.
Yesterday, just out of curiosity, I decided to test the Kenko with the 100-300. I mounted it on a tripod in my studio, set the aperture at F8 and did 3 test shots at 100, 200,300mm (150,300,450 with the TC). My test target was my Silver 77 Limited. I was shocked at how good this combo is. Better than the matched Sigma. I then decided to test the Kenko + 100-300 at 300mm at each aperture from wide open f4.0 to f32. I chose 300mm because I am most likely to use it at that FL. I was very pleasantly surprised. Wide open it is a touch soft but very usable. A little USM brings it right in. Between f8.0 and f16 you would not know a TC was used, it is that good. Beyond f16, defraction effect sets in and you can see the image soften. I am very sorry, but I deleted the test shots when I finished.
My conclusions are the Kenko you bought is a bargain, A more expensive TC will not get you noticeably better images. The key is too use it with top draw glass. So save your money and buy a better lens to go with the Kenko TC.