I'm referring you to maxconsole cause I gave up on my PSP long ago and have not followed the scene recently. I do not know enough about it these days as, when I stopped reading up on everything, you couldn't play games that required 2.6 firmware on 1.5 PSPs. I gave up on trying to do that very same thing and upgraded to 2.6 and never looked back. I did read on maxconsole though that you could do it, soon after I upgraded (ack!).
Basically, I'm referring you to maxconsole, not because I'm trying to get rid of you, but because 1) it'd be cool for you to get some experience doing research like this, and 2) I don't know what to tell you other than refer you to the people who know the most about it.