Here is some reading that might give you some options.
Joysen for joystick sensitivity
JoySens 1.3 released, PSP Slim Hacks - PSP Hacks
tutorial on psp analog stick adjustment - Google Search
Unsure about joysen,never used it so be cautious & if you make an adjustment take note of it,so if you have a problem you can always go back to were you started