I too have an 540iA with non-M rear bumper.
I went with the Magnaflow 14815 and I liked it but it was still a little too quiet but better for sure. I then decided to remove the massive Y- resonator. It definately sounded much better outside the car but the resonance at around town speed got too droning for me personally. It was fine at freeway speed, but this being my commuter, it got a little old. I then added an Imco aftermarket resonator in its place but it did not do much, so that was a waste of money. I did keep the original resonator and had it re-installed and I am fine with it now.
Hope that helps.
When is the next road trip?
2002 540i/A Topaz blue, bone stock.