For the Z800 I'd personally deactivate the LSI SAS "device" in BIOS, under the Security tab, Device Security, before the clean install. I'm guessing you'll never be using SAS and there have been some reports of difficulty with the drivers for that so if you make the device invisible to the OS this way you should not have to deal with any of that. I don't have a Z800 to work with, so that is an educated suggestion.
For sure first make sure your BIOS SATA Emulation is set to RAID + AHCI (or if that option is not present setting it to RAID will get you the same result. Agree with first updating to that latest 4/16 BIOS from HP.
Looking forward to hearing about all the fun.