I've never done that but the secure erase via the bootable USB drive option worked fine, and I would think the FW update would work fine as well. I'd hardly call it lazy wanting to bypass installing Windows, etc, and creating a RAID volume again, for a 30 second FW update.
I was able to do the secure erase in RAID mode, although I did not have the 840 Pro's in a RAID array.
One thing you might encounter using the CD or USB drive FW update method is the UI will appear as an unreadable mess when using a video card (it was fine using the Intel IGP!) If that happens, you can exit the UI and run the FW update from the DOS command line. That is described in the Troubleshooting section of the Magician's Help option. I had to use the DOS command line method, which actually loads a font apparently, and it worked fine.