If 2.10 is really the one that brings anti-downgrade verification, then it reveals a bit more about Sony's fear of RSX hack... (2.10 is the one which countered RSX hack).
If the 'upgrade twice then downgrade' trick really doesn't work then, for sure, that means an emergency lock up procedure (probably based on efuses) has been activated in 2.10 and above, and the 'failover' ability has been silently dropped. So, for 2.10 or above, Sony rather likes letting PS3's get bricked by accident than allowing the RSX hack to reactivate.
I think this is the only logical explanation to this 'panic' mode activation on Sony side...
The RSX can probably be used to put down the entire PS3 security... And Sony remains completely silent about RSX to not give any clue on this (probable) fact. After all the 360 GPU's helped a lot to turn it into hacked platform.
If this scenario is true, then the way to go is :
- remain below 2.10 at all cost
- hack ps3 completely under 2.10
- create custom fw's based on more recent fw's for game compatibilites, spoofing them
(a nice and long way to go...)