The existing sounds do the job. They're functional, but they aren't very pleasant--or realistic for that matter. The tire noises sound like something out of an old Road Runner cartoon--real squealing tires don't sound anything like that. The engine in the FXO DOES sound a bit like a flat 4, but the LX6's mill doesn't sound a thing like any straight six I've ever heard.
Your Harley sound does sound a lot like a V-twin cruiser, but not a Harley. I've heard Harleys and they're not that smooth. Your first 911 though is almost bang-on.