From what Ive understand downgrading the PSP is not possible. On the other hand I see alot of people experimenting on how to downgrade this with the official upgrades. Hmmm...
As a software developer myself I do have some problems with the logic here, since we dont accually know if theese are 100% upgrades of the complete BIOS or if its patching the BIOS. I remember when I upgraded my v1.0 (Man had I known...) to v1.5 the download was around 23 MB I think, happy that I had gotten the network to work I testet again and upgraded to the v1.51. However, I thing this upgrade was around 11MB.
So - my point - since people want to downgrade with the official upgrades. To me it seems that the v1.51 upgrade only contains the changes from v1.5 and not the complete system, meaning that a downgrade would most likely never work at all... If both the upgrades were the same size that would indicate that it was a complete system...
Just some thoughts after searching for hope... :)