Very good first post with lots of info. Well done.
The only thing I'd like to comment on is your experience with the Z950 w/MOD. Something was definitely wrong with that one if it bogged down, or didn't get rid of the clippings. I have one that I bought new this spring, and while I have had a hard time getting it to cut as clean in one pass as I'd like, it seems everyone else is having QOC issues this year as well with the drought, so maybe that's all it is because now that we've had a few rains, it is cutting pretty good. As far as cutting thick tall grass, mine will walk right through it easily, and leave a very evenly dispersed layer of clippings that totally disappear on the second pass. Unless it was new, I'd guess it had less than freshly sharpened blades on it and/or a clogged up underside of the deck.