The bigger G scale engines use bigger speakers and enclosures. The boards have the same output I believe. Yes, they can sound much better if cranked up. I usually keep the sound turned down so they actually sound just about the same. MTH started putting better speakers in their O scale diesels so they are much closer. Some of it comes from when they are turned up louder. I like to hear the train (the wheels really) so I now how it's doing.
The first time I heard a MTH G scale engine was at LBC hobby. He had the Hudson just in and the volume was up full. I was actually pushed out of the room it was so loud. I never hear a toy train with that much sound before.
The board set originated from a Amtrak Dash 8. I changed the file to EMD. I started with the soundset from a GP38-2 that was great. The number boards weren't lit? So I switched it to a GP40. I should have probably tried the MP15AC soundset. I figured because it was RK, it may not have all the lights activated??