Here is my 0.02:
- with main (hammer) spring you can go only as low as reliable primer ignition allows it; most people stay with 13# springs.
- with recoil spring, you should stay as low as you can to allow reliable case ejection and ammo chambering; lower rate spring prolongs slide stop life too.
Too high of a rate may cause short stroking for slide. If you go too low, you may get ammo chambering issues. Most people go with 11-15# range. With lower power ammo one will need lower (11#) rate spring.