I just happen to be testing my Tammy TC 1.4x MC4 with my DA55-300mm today...again. My findings are the same as last month. The DA55-300 is sharp enough at 300 but when I add the TC the sharpness suffers just enough that I feel it crosses the line between good and not so good. The combination works fine, AF included, as long as a few extra re-seeks don't freak you out. I know from comparisons with others that my lens is a good copy but I dont have any comparison for the TC. However, I just finished using it with a Bigma for 8 days and liked the results so assume the TC is fine. It also works well with my 800mm mirror lens and a few other cheap manual 300mm lens. I will come back and post 2 comparison pics in 15 minutes. Your mileage may vary but my testing has been consistant.
DA55-300mm+1.4xTC on left at 100% Same shot and lens w/o TC on right at 125% so its almost cropped the same size (can't do 140%)
Yes, pic is a little small here but you can see the difference and its not the ISO as I changed that to keep the shutter speed up but have done this test many times. Just happened to have a couple shots still warm in the camera for you. Hope it helps you decide.