I hate extenders.
Don't get me wrong. I use them, but I hate them. When you get to longer focal lengths, everything is conspiring against quality: haze, vibration, shallow depth of field, lens quality. The extenders add another negative to the equation. A few years ago, I did some extensive experimenting with my VII extender on my 100-400 and found no usable increase in resolution. I am still using my 1.4x II, but mostly because I can't imagine that the VIII would be so much better that it would change my general disdain for extenders...again, the extenders that I use somewhat frequently on my 400 f/4 DO.