AP - I think the quietness factor is more down to how little noises the car generates rather than it shutting out environmental sounds entirely.
There is little engine noise, save at WOT - I actually wish I could hear it a bit more. Tire noise is nearly non existent, as is wind noise. This car has a very slippery Cd for such a large vehicle. But if someone is shouting at you from outside you will hear them. And Ron is right - there is pretty much universal agreement that the 430 is quieter. The 460 has double paned windows on the front only, the 430 has them in back as well.
I can't think of a single review that suggested this car was particularly engaging to drive - so that may have been an unreasonable expectation on your part. The GS is FAR more engaging, but wholly lacks the gravitas of the LS.
I could be happy with either, but they are very different rides.