I'm pleased (and quite surprised) to report that using the XB1's free cable between my XB1 and my AVR appears to have solved by video passthrough problem. I really didn't expect this, given that I was replacing a pretty nice Audioquest HDMI cable with the free one, but it clearly and substantially improved the passthrough video quality.
I now have my Tivo XL4 plugged into my XB1. With my 100" front projection setup, there may be a subtle loss of sharpness by doing the passthrough. But it is, at most, subtle, and at times, I felt like there is no sharpness loss at all, so it was hard to tell if it is my imagination. Previously, when I used my Audioquest HDMI cable between the XB1 and my AVR (actually an Onkyo PR886SC preamp), there was a definite loss in sharpness and a very noticeable loss of brightness. The PQ was unacceptable, and I had given up on running my Tivo through the XB1 until I tried this fix.
High five later to those who first discovered this counter-intuitive solution. First time I've ever see a cable change have such a big impact.
After 7 Years With a JVC RS1 - Starting to Get the Upgrade Itch