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.. Sadly I now own $500 worth of tele converters and only use the 2x.
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. Add the tele, 300*1.4= 420mm When you need it the D7100 has a 1.3 crop mode, 420*1.3 = 546mm F4. Well worth the $250 price tag.
Summary: Focus is very fast even with extension tubes + this 1.4x..The result was better without a close up lens..I tend not to use a close up lens so I can use the sigma ring flash EM-140..With Vivitar extension tubes+1.4x, there is no drop of quality except a little loss of light but even so, the autofocus is surprisingly very fast even I stop down to F16.. I have sony A7r with LAEA4 adapter..