The 1.4x came as a bundle with my 100-400mm and I must admit that whilst the IQ does soften a bit at 400, it is still very acceptable for the extra reach. When I bought the 50-140mm I was amazed at just how good the !.4x was at all focal lengths.
A 2x came up on EBay last month, as new, so I offered a buy it now price and £170 was accepted. It is superb on the 50-140mm and again, up to about 300mm on the 100-400mm.
Twixt the two the 1.4x is the much sweeter but the extra reach on the 2x does come in handy.
I did a bit of research before keeping an eye out for a 2x and found this article to be very useful:
Now THIS is a real odd-ball. – A guy who is completely and almost exclusively into shooting small prime lenses on the narrow streets of Århus in Denmark, suddenly reviews a 2 x tele converter…
jonasraskphotography.com