If you are just trying to eliminate the drone at cruise and like the sound you have now here are a few things you can try.
Make sure the trunk seals well, any holes in the floor or leaks in the trunk lid gasket will make the drone louder.
If you don't have sound deadening, get some (I have been using the foil faced bubble wrap from Home Depot and Lowes, cheap, better stuff is available, but I gotta start somewhere and for what it costs it works good enough).
If all that is good, try a different exhaust tip. We have pencil tips on one of our cars (FWIW 2 1/4") and we had to extend the pipe quite a ways past the back of the car to eliminate a resonance like you describe (you could try the same). A friend has a similar setup with what I call meat grinder megs. They are straight till about the last inch or so then flare out like the 'meat grinder' velocity stacks. It seems to do a better job getting the noise away from the car.
You could try a small glaspack just forward of the bumper as a resonator.
The H pipe will make it quieter, it will also take some of the rasp and rap away and make more of a gurgling sound.
I have Dynomax super turbo mufflers on my A with a bent 8 in it (OT I know) they are quiet at cruise and have a nice tone when you blip the throttle (or are screaming up through the gears) and don't drone. We mounted them as far back in the pipe as we could.
Are you sure it's exhaust drone? I have a friend that chased an 'exhaust resonance' problem for over a year, before he figured out it was the intake that was droning (not too likely in what you describe, but woth checking out).
This is not conclusive or end all be all, and I'm no acoustical engineer. It's just stuff I have tried or seen/helped friends try.
