Can you downgrade a baseband? I'm on 5.16.02 and would like something unlockable while avoiding the iPad 6.15 BB to prevent killing my GPS.

OK so I've read so much about this in the last 2 days that I'm confused. I have an old-bootrom 3GS which runs 4.3.5 and BB 5.16.02 which is not unlockable. My ultimate goal is to have an unlocked phone, an to jailbreak it is a secondary objective. My understanding is iOS 5.1 is jailbreakable (even untethered for my device) and if I flash 6.15.00 I can even unlock it? Does 5.1 give you a different BB and is that why you need to preserve it? I read the risk with loosing GPS but I've also read somewhere else that it only applies to newer bootrom.Can anyone verify that? Now the big question is why would I not want to go to 5.1 with 6.15.00 preserved?