I can confirm from first hand that the 2.50/2.60-1.50 downdater does have side effects. My Burnout Legends was not loading savedata on my 2.60-1.50 downgraded PSP or my mates 2.60-1.50 downgraded PSP. But on my cousins 2.00-1.50 downgraded PSP it was working perfectly! I upgraded to 2.00 and downgraded back to 1.50 using MPH and now my PSP loads savedata off Burnout Legends perfectly. Also my memory stick and UMD's load quicker than before. (And my UMD is making normal sounds again! Yay! :Punk: )
I must note that I got my 1.50 files for the 2.60 downgrade from PSP Hacks.