Summary: I tried this with a Sigma 150-500mm on a Nikon D7100 DX and a D800 FX. Yes, you will get 40% magnification but the loss of image quality was higher. I could crop in post processing and get better detail without the TC 1.4 than with it.
Summary: Just buy the 2x and don't waste your time, unless you have a specific need for just 1.4, such as needing more light that the 2x does not afford. I bought this to use on my Sigma 150-500, as the 2x doesn't allow enough light for the auto focus to work..
Summary: Using it on the SIgma 70-200 2.8 OS and love it. You do lose a stop of light making the F2.8 a F4 but still a good way of getting a 400mm F4 for a small price. I use it on the Nikon D7100, when adding the crop modes on the long end. The 200*1.5 = 300mm.