According to the picture, PS3 Slim CECH-250xA are not always hackable.
There are different release dates using the same Serial number.
CECH-250xA released in 2010 are hackable (initial and most lower installable firmware : 3.40 or 3.50)
CECH-250xA released in 2011 or newer are not hackable (minimum downgradable firmware 3.60! you need to be on 3.55 to install a custom firmware)
But there are maybe another way to install custom firmware without downgrading : Can you patch your NOR dump on your computer to Rogero 4.40 and flash it back in PS3 ?
I don't know if it's really possible, but isn't what BwE NOR NAND Validator/Patcher is doing ? I don't understand it very well, and nobody seems to report using the patch feature.
Maybe Devin could test it, if he didn't sell his console/E3 Flasher yet ?
Or is the patch needed only to validate the dump?
edit:
http://www.ps3hax.net/showthread.php?t=54377
http://www.betterwayelectronics.com/
http://www.ps3devwiki.com/wiki/Validating_flash_dumps#BwE_NOR_Validator