Thanks to VisionLight......my 2nd day.
I don't normally use the full digital zoom, only the 2.0 TC and the 1.5 TC. The TCs appear to have some extra in camera processing that gives better results than using the Digital Zoom. Canon states something to this effect in its literature. See my test from last January here:
Andy, as a new owner, you may be interested in going through the whole thread. It turned into an interesting discussion.
I've been playing with tests like this since the days of my Nikon 995. Not much has changed. The end results are like splitting hairs really. If Canon claims any special processing, it's likely just some up sizing and sharpening with slightly different metering applied at the point of the shot being taken. I could see metering being different and perhaps advantageous, but overall, I'd still keep it limited to 840mm and crop / enlarge if needed.
Here's the same test, 840mm interpolated to double the size, then a 100% crop taken vs Optical vs 2.0TC cropped in relatively the same area. 100% sizes in both 600x400 samples taken.
Note my new Screen Name has changed due to some privacy issues I was having to deal with, but I'm the same old long time member. Email or Message me anytime though.