No. The PRL cannot be changed. Only updated through the Settings menu.
The PRL has nothing to do with internet speeds. Its your preferred roaming list. An I'm sorry, the prl can be changed to anything you want. If your still with sprint though, you may want to leave it. They monitor what you do, towers you access, etc. If you changed it to another CDMA number other than sprint you could get busted.
I'm on virgin, I've changed mine a few times.
The only thing it may do is help connect to better towers. The radio is what controls your actual reception. And yes, that can also be updated. Its a long process and can brick your phone if done wrong.
Do a search, changing prl and radio are easy to find directions for...
As far as updating through menus, be careful. It can remove root if you update the wrong items
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