Thank you for your patience and understanding. This is the response from engineering:
You should not experience any issues using drives with a mixture of OEM firmware and the Intel generic firmware.
Each manufacturer who provides Intel® SSD’s with custom firmware makes their own decision on implementing firmware updates for their products. Our Intel SSD Toolbox does not have the ability to update the firmware on OEM drives. Please check with your system manufacturer on their plans for SSD firmware updates.
Thanks for the answer.
In meantime, I've got no reply from HP but have purchased additional 4 HP branded drives to finish creation of RAID storage.
PS. It would be nice to have a choice, since Intel SSD Toolbox recognizes it as DC 3700 drive.
I'm not asking the other way around - to replace Intel generic firmware with HP one - and understand the implications of doing that.