The hardware inside the phone. the distance it is from the transmitter and the environment in between is what determines your signal strength. The ONLY way to increase it or decrease it is to build in an RF amplifier or attenuator. That isn't something that can be done in firmware or software.
To many people get WAY to anal about what their different "Meters" on their phones are telling them without knowing WHAT those "meters" are actually doing.
The actual signal strength does not change from one ROM to the next. That would be impossible to do at the ROM level. The only thing that can change is the algorithm that takes the dBm signal strength given to it at the output of the transceiver and "translates" that into a readable quantity. So, if the ROM "samples" the signal more times per "x" cycle than another, the "signal strength" will be more averaged over a longer time period. The longer "sampling" time, the less averaged it will be. Then you have the unknown of HOW the ROM developer uses that information to write to the screen.