After successful migration from C-5 to C-8 here are some things I noticed which have all since resolved themselves in the C-8. The C-5 was powered down after I did the cloud backup, so it was not a factor.
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When the C-8 (@ ver 2.3.5 platform) first was pulling in the cloud migration from the C-5 I had a kind of light show. ;-P. My Leviton Zigbee wall switches which we on (non dimmer) started going crazy. Turning on and off very rapidly in a random pattern. I got annoyed so I went over and physically hit the off switch on them.
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Asking to do a software platform upgrade while running 2.3.5 download went extremely slow and ultimately failed. Tried three times. After giving up, I asked for a reboot and tried again. This time the download went very fast and succeeded in upgrading to latest 2.3.6.
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After rebooting into 2.3.6 on the C-8:
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My Sonoff ZBMINI switches, the first gen which require a neutral wires, decided to drop off the map. About 12 of them. Doing a Zigbee add new device operation brought back 3/4 of them (found previously paired devices). Doing it again, brought back more. A third time, seems to get them all. I hope they stay connected. So far so good.
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I had a zigbee Light Group 2.1 composed for some dimmers. 3 smart bulbs and a Leviton zigbee wall dimmer. I had noticed a new option in the grouping was set called "command mixed devices?" was set. I don't remember this option and I don't remember setting it. It was working fine with the option. Turning off the option however, made the Leviton wall dimmer not cooperate with the group device. So I switched this "command mixed devices" back on. Not sure what it does exactly, but kudos to the Hubitat team to turning it on for me. I assume by comparing what kind of devices were in the grouping?
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Played around with the Zigbee rebuild option and TX radio power and after giving it 24 hours the mesh seems to be strong again as it was when I had the C-5. Things "seem" faster to respond. I have about 120 zigbee devices. Nice.
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CPU load shows the hub 2x higher then when I had the C-5. Memory usage also seems to be a bit more in the C-8 vs the C-5 when first starting up. However, the rate at which the memory drops seems to be a lot more stable. Seems to be holding at the same level once it stabilizes. Hopefully, this will prevent my need to reboot daily like I was with my C-5 due to memory leaks. fingers crossed
Overall, I am happy with it so far. I'll probably keep my C-5 around as a backup, just in case I need to
buy another C-8 and need to do the migration again. Lock my C-5 in an environmentally controlled safe place. 
