Right now the module is running at 115200 bps, and I'm worried that some byte loss might occur between the Arduino and module. If I lower the bit rate to 9600 bps (the lowest I can select for the module I'm using), would this issue be averted?
The potential for serial buffer overrun has far more to do with what the Arduino is doing while it is not reading serial data. The Arduino can read data orders of magnitude faster than it can receive data, even at 115200.
An additional note: I'm planning on using the serialEvent() interrupt to drain the incomming byte buffer by storing the packet data bytes into a byte array in the RAM as soon as they're received. Then doing any logic afterwards.
The serialEvent() method is NOT an interrupt. It is simply a convenience function called after each execution of loop, to allow you to move the reading of serial data out of loop(). It does not interrupt loop() when serial data arrives.
You need to talk more about what this mysterious "logic" is. It, apparently, will be what influences how fast you can deal with serial data.