This is caused by a compatibility issue between Pheonix and the "blockchain" driver from AMD, I no longer have that issue using 18.3.1 in compute mode.

Here is what I found:
As of 24 hours ago, I switched from the AMD blockchain driver (AMD will not update this driver or support it, this driver will not work with more than 8 GPUs) to the latest AMD driver v. 18.3.1 and switched to compute mode. Here is what I observed:

Temp and fan control is now the same as Claymore. Ambient temp 22C, GPU temp 45-55C at 0-55% fan (my setting is -tt 55 -fanmin 45 -fanmax 70). This is on optimized BIOS settings (-cclock 1150 -cvddc 875 -mclock 2150-2200 -mvddc 850-900).
Everything seems to be running stable with the pool reporting avg effective 223MH/s.

My conclusion is: Dump the antiquated "blockchain" driver from AMD and install the latest 18.3.1 (do not forget to switch to compute if you are running dedicated mining rig, this has to be done to every card and restarted each time so grab a coffee. It figures AMD didn't include a way to apply this change to all the GPUs at once, that would be too logical)

Here is a summery based on my test (8xRX580 8GB):

PhoenixMiner                   Claymore
228MH/s                              225MH/s                       Adv Phoenix
0.65%  Devfee                     1-1.5% Devfee              Adv Phoenix
Stability                                                                   Adv Pheonix
Power consumption                                                  Adv Claymore
Fan control                                                              Adv Claymore
Userbase                                                                 Adv Calymore

These are the most pertinent parameters impacting the bottom line.
Feel free to add more. Hope this helps people to decide.           

So after reading through everything, it would appear the problem lies with the AMD driver. I am using the AMD blockchain driver, and apparently that will not work with more than 8 GPUs (cause Windows to not boot up, kind of dumb given that mining is the only reason why multiple GPUs are needed in the first place.). Guess I have to uninstall the AMD blockchain driver and re-install the 18.3.1 (there is no 18.2.3, it goes from 12.8.1 to 12.3.1)standard driver (and switch to compute), and see if this works.

Please enlighten me on how it is you were able to break the 8 AMD GPU barrier in Windows 10?

Thats just it, I didn't. The OC settings are the same on all 5 GPUs across the board. 1130 Core, 2200 Mem. Undervolt to 850 on core and mem.
I have since then lowered the mem clock down to 2150, as this seems to be the most stable for this type of GPU.

The hash rate reported by Claymore seems flaky at times. I can started once and it will report 24.5 to 27.5 MH/S, just stopping and restarting the miner will change the hash rate to 27.5 to 29.5.

I took a photo of this so people wouldn't call me a lier.

All 5 gpu are MSI Armour RX580 8GB

All have exactly the same OC, BIOS mod

The 2 with Micron mem are running ~29.8 MH/S

The rest have the Hinex mem running ~27.8 MH/S

But this 1 freak cpu with Hinex is somehow running at 45.5 MH/S @48C

Alas it wasn't meant to be, it only lasted 10 min before Claymore reset itself and the freak went back to its old self. lol

Any chance CryptoNight can be added to the duel miner?

Has everyone lost their mind?

Why would anyone pay $600-$800 for something that costs $180.00?? Are people that blinded by greed??