Strongly agree on the radio power thing... Assuming you have a healthy mesh of (preferably ZB3.0) repeaters, lowering the radio power was one of the things that very dramatically improved my zigbee mesh issues when I moved to the 8-series hubs. I use the lowest power setting (4 I think? I'm at office so I can't verify)
ETA -- for OP, I had 3 key things that finally fixed my 8-series hub zigbee issues...
1.) Removed any old ZHA1.2 repeaters from my mesh (in my case, 2 old Halo smoke detectors that were gumming things up with my 8-series hub) -- moving to all ZB3.0 repeaters helped a lot.
2.) Moved to channel 25 (from 15). My Hue has forever been on ch 20, so that stayed as-is. I didn't have any direct (wifi or other) conflicts on ch 15, but the move to ch 25 made a big difference in performance for me... For whatever reason, the 8-series hubs often don't like ch 20.
Edit #2 -- verify that ch25 is clear in your local spectrum before just switching to it-- I didn't mean to imply that ch25 itself is a cure-all; I just mean that looking for a different clear channel option in your own local spectrum can be wise to consider (but still avoid using ch20 with 8-series hubs)
3.) Knowing I had a strong mesh of ZB3.0 repeaters in place (position-wise), dropping the ZB radio power down to its lowest setting was the final thing that really helped.
