Last night I had the chance to try and downgrade a Slim Ps3 (JSD-001 board with Spansion NOR) version 3.70 to CFW 3.55 using Dospiedra’s downgrade v2 patches.
I managed to get a clean dump of the NOR using my ProgSkeet (latest QT port used), created a patched Downgrade.bin image
using the 6 patched files from Dospiedra and a hex editor, then I flashed the image to the PS3 NOR, dumped the NOR another
time and verified that all was written fine.
Then I continued with the normal downgrade procedure, go into service mode, update using the 3.55_no_check.PUP, then exit
service mode and all was good, it was back into CFW 3.55.
Here the problems started, I tried to start a game and I got the famous Trophy error, so I decided to update it to my
personal Modified Firmware, so I started the update and after installing the firmware, the PS3 turned off by itself,
I tried to turn it on again, It turned on for almost 2 or 3 seconds then turned off completely ( No Screen Output, No Red Led either )
so I knew-ed at this point that the PS3 was Bricked 
N.B: this was never mentioned in any of the downgrade tutorials floating on the net, although this is a very important point
to warn the users who are downgrading their Ps3 machines not to update using usual Modified firmwares unless the firmware
have the LV1.self file patched to Disable all checks, anything else will result into a Bricked PS3, and this is not good at
all especially if the Hardware flasher used to downgrade was removed from the Ps3′s NOR or Nand Flash.
After doing some research and discussing the issue with my friend eussNL <– a wiki by himself 
I realized what happened to the Ps3, after the downgrade procedure, the machine’s syscon still had a version > 3.55
(3.56 or higher) and this needs a Patched LV1.self (checks disabled) in the NOR for the PS3 to be able to boot fine, and when
I updated it to my own MFW, the LV1.self file in the NOR was replaced with a non-patched version (checks enabled) and the PS3
detected the higher syscon version (3.56+) and Bricked.
To Fix it, I had to re-flash the NOR again with the patched Downgrade.bin image (to get rid of the patched LV1.self)
then the Ps3 was fixed and booting fine again.
At this point, the Ps3 was working again, but the Trophy problem was always there, so I prepared another Modified firmware with
3.70 spoof, Privacy Patch and this time the LV1.self Checks Disabled
(the patches were provided by eussNL too so Credits here goes to him),
then while still having Progskeet soldered to the NOR flash, I updated the Ps3 with the new MFW, everything
went fine, and it rebooted fine into the XMB, did some tests and the Trophy problem was gone for good and all games working fine.
For all the users who had successfully downgraded their PS3 machines to 3.55 again, I share with you my MFW with Lv1 Checks patched to bypass the 3.56+ syscon version and prevent any brick after updating to it, and to get rid of the annoying Trophy problem encountered after the usual downgrade procedure.