"I have not yet found much use for my 1.4 extender on my 300 mm lenses - I seem to need the bare lens or all the reach I can get!"
I guess it depends if you shoot other than birds, say larger mamals. Then it depends how wild they are or how much effort you're willing to put into stalking them. Since I've mainly been shoting birds and often smaller ones, I was always reluctant to head out without 300 X2 and typically felt short changed if I used X1.4. Only when I know I can get close to a bird do I use 300. Now, for bugs or butterflies, 300 or 420 with 36mm extension is very handy! In fact I'm thrilled with that.
Sometimes moose will come so close that even 300 is too much but not too often. Case in point, sitting in brambles squirming around trying to frame this guy with 300 X2. Uncropped
6D 300 X2 400th F5.6 ISO 2500