Reading back , I think I see where some confusion was. The "checksum error" is when you reboot the very first time after flashing the BIOS, and it says "F1 to configure" or something to that effect.
Renthx is correct, it always does that. (I like it, no chance of missing the chance to get in to SETUP and having windows load up before getting in there to change important things, like S3 suspend mode)
Once you save your BIOS options, the "checksum error" goes away permanently.
I flashed using the windows BIOS util, boot block + nvram and "clear cmos" checked.
It worked fine, went to 100% and asked for a reboot. Nothing weird.
Rebooted, loaded defaults, set up some options, and checked MEMTEST with and without USB legacy, everything was fine
Then powered down, cleared the BIOS (red button), and loaded defaults/reset my options and repeated the above tests. (I like clearing the BIOS before loading defaults, no particular reason)
I did change the boot order, and that saved fine as well.
Booted up windows, no new devices were detected.
So I can't say there was any issues at all updating the BIOS.
Now, there IS a small issue with LEGACY MODE. It appears your RAM Bandwidth takes a small hit if you turn this on. Maybe it's part of the memtest fix? Dunno.
The bandwidth shown by MEMTEST:
USB LEGACY MODE OFF:
L1 20492
L2 5091
Mem 3255 MB/s
USB LEGACY MODE AUTO with a USB mouse plugged in:
L1 20492
L2 5091
Mem 3152 MB/s
100 MB/s is not a big deal, I guess. But it was an unexpected side effect.