I've only found it possible to obtain a usable image in post-processing using added contrast if I overexpose by about 2/3 of a stop or more, and have a very well balanced histogram on all of my RGB channels. I'll almost always prefer using a bare lens and cropping to my desired focal length in post than using a TC and boosting contrast. That's just me though.
Well we all have our preferences I've never had to add extra contrast when using the 500L II + 1.4x III simply because of the extender. I'd still expect that if someone posted images taken with a bare supertelephoto lens and the same lens with a 1.4x extender (mark II or mark III) it would be very difficult to tell which were taken with which setup, but that's just my personal experience.