Arthur Morris has some videos on YouTube about using Extenders with Canon lenses. They are worth watching. Steve Perry has some videos about using Nikon TCs on YouTube also. Neither of these will address your lens. You will want to maintain autofocus unless you are shooting stationary objects. That will limit how much TC you can use with your already slow lens.

I use a 2x Extender with my Canon 70-200 f/2.8 II and largely get results that satisfy.
But I am also using it on a 1Dx mkII body which has excellent autofocus system and high iso capabilities.

My advice would be to watch the videos and rent or borrow a 1.4TC to do your own testing.