I'm going into cranky old man mode here:
1. PRL = Prefered Roaming List It's just that, a list that tells the phone what towers in what areas to use when it can't log on to the home system. Unless you are roaming, how could it possibly improve signal strength. Plus, if you are using an outdated list you are likely to not be able to connect to all the available roaming choices.
2. Signal strength is a function of the hardware and the radio software, which isn't part of the ROM. How that signal strength is displayed is a function of the ROM and software. Bars mean nothing. You could cab up a signal strength meter that always made it look like you had 6 bars of service but it wouldn't improve reception. That's what CrApple did with their iph*nes and they eventually got called out for it.
I can sit at home and watch the bars on my phone go from 1 to 5 without moving it at all. There are lots of factors that effect signal strength, don't assume that a specific PRL or a specific ROM makes a difference.