Thanks, Phil! Those look quite nice. Keep them coming.
I'm getting dizzy considering all the possibilities, but I keep coming back to your setup as the most quality and flexibility for the money. I'm afraid the Sigma 50-500 would just be too big for much hand-holding and I'd hate to give up zoom and IS with the 400mm f5.6 prime. And Canon primes with more reach - well they are out of my price range. (But then again, I've just ordered at Gitzo G1548, so I can mount a tank.)
I'm even considering giving up digiscoping with my Kowa TSN2 and Coolpix 4500. As Andy Bright said elsewhere, birding and bird photography don't mix well. And I'm finding myself more interested in photography than birding. I'm hardly ever satisfied with ds quality and fussiness and I find that cropping pics from the Rebel gives me nearly the quality as digiscoping! With the reach you have, there is even less cropping to do on the long shots.
I've found the quality of the 75-300 IS to be good with enough light. So if your x2 quality is equal, that's not bad at all. Like you, I figure I'd keep the 75-300, especially for when my wife wants to take pictures as it is not too heavy for her (with Tamron x1.4 TC).
RonL