Interesting discussion gentlemen.
The way I see it, and I could very well be wrong, is this:
A zoom like the 100-500 Bigma or the new 150-500 or the Canon 100-400 will give you at the most 500 x 1.4 = 700mm. IMO, the zooms are not sharp enough to allow 2X TCs.
On the other hand, a 600mm ED astro doublet will take a 2X barlow or a good 2X TC and still provide quite good results, as good if not better than the 500mm zoom with a 1.4X TC. 600 x 2 = 1200mm. In fact it is quite a bit more if you account for the spacers.
This is about twice as much in terms of magnification or reach. Quite significant.
I have a Canon 100-400mm and an Astro-Tech 560mm and there is no comparison in terms of reach - I'll take the scope any time if I need reach. However, the scope is a big bugger, heavy, slow and entirely manual. But when I don't need the reach, I use the Canon Zoom all the time. So much easier and faster !
Well... this what happens when you don't have the budget for a 600mm lens... And anyway, when birding for water birds, I need a spotting scope !
By the way Skimmer, a 1.4X Kenko TC is fine. Will it take better pictures ? I don't know - it depends how well you'll be able to handle your setup. You'll have to be patient because there is a learning curve - at least it was the case for me.