@Hooves and Bearcat,
I take it you never read the pilot reports of what the machine guns sounded like from their own accounts? I believe the majority said the machine guns were close to the sound of fabric ripping (in many pilot accounts). There's even a video posted in the TF forum taken from live gunfire that almost virtually identically masks the sounds recorded from gun fire.
I think many people forget that not only do you have a 1000+ HP engine with exhaust shorter than my forearm sitting directly in front of you, you also have a helmet that permits you to talk which tries to mask most of this droning sound away in the 1st place. Then you are further masked away by being in an enclosed cockpit.
I'm not a pilot by any means, but I've gotten the opportunity to ride in some authentic warbirds and wear a communication device. If anything, you'd probably be pretty disappointed by what real sounds you actually hear while doing so. If you think the engine, let alone machine guns is going to sound anything like what they would from the outside, or what they were if you were firing a gun with no other environmental sounds, then it's no wonder you have the impression the sounds are pretty bad.
The TF guys weren't going for a machine gun sound like some other sims try to do, they were going for the sound you would actually hear if you were strapped in the plane, canopy closed, with a sound muffling headset worn (such as real life and the pilot reports/recordings suggested).
I gotta chance to ride in a 2 seater P51 and I can tell you the only thing I heard of that merlin was an annoying drone, sounding absolutely nothing like what you would hear as an observer outside the plane.
As far as the modded engine sounds, they are pretty spot on:
Here's a flyby -