I agree with you. The only reason the reach and pixel density of the 80D doesn't help me much is that I am not cropping. If you need to crop, the 80D will be a big improvement over the 1DIV.
Low-light shooting involves two factors: high-ISO noise, and autofocus.
With respect to AF, the 80D is a clear winner. The 1DIV only focuses down to 0EV, which I used to (until 6D) find acceptable. The 80D can focus down to -3EV, which it does uncannily well. FYI: when I am shooting in low-light, I am using fast primes. That makes a difference. All Canon cameras focus better at f/2.8 and faster.
I don't think the high-ISO noise differences between the 80D and 1DIV are worth mentioning. I will readily use either at ISO 6400. I find the 80D usable at 12,800, but it requires spot-on WB and ETTR (and raw shooting) for best results. As mentioned above, that is a compliment to both cameras. An aps-c sensor equaling an aps-h sensor is good news. A 7 year old camera rivaling a new camera is equally good news.
All in all, for the way I use my AI-Servo camera, the 80D is as good as the 1DIV. Or the 1DIV is as good as the 80D. For One-Shot, I am using my 6D.
And last but not least, I am healing well. Thank you. My last follow-up with the surgeon was Wednesday. He cleared me to go back to work. I have accepted a new job in PACU [recovery room] that starts 2/6/17. I started resistance/strength training yesterday. I can now handle a 6D/24-105, as long as I am careful. I assume I'll be able to shoot my 1DIV/100-400 again by late Spring. Thanks, again.
Well that's good news (about the 80D and you).