I know Colin Mcrea Rally 2005 requires v2.0 but it is possible to run on 1.5 PSP by using WAB Changer program to make it look to the program that you have upgraded to v2.0
somehow i dont think that will work on newer games such as GTA and Pro Evo 5....
think it is necessary to wait for patched versions of the games to become available on web (think there is already version of Pro Evo 5 somewhere)
ultimately think upgrading firmware will be only way to play latest PSP games in the future....thanks sony!