I have a the dual muffler Borla and had a good amount of drone at those same ranges too. I can only tell you that after about 3000 miles 90% of the drone in "D" has gone.
I hear what you're saying. It was such an annoyance I didn't used to drive in "D", but now it sounds great in both D and DS.
One thing that I did was found that the metallic resonance/vibration was being amplified in the rear storage bin between the "two" rear seats. I put some Dynamat (sound & vibration deadener) in the bin and that killed a lot of it.
Honestly, when I first installed the exhaust it was one of my least favorite mods because of the drone. Now I LOVE it and it seems to get better and better. Still think it sounds amazing in DS. Love the burble tooling around in DS and have gotten several compliments.
Don't be afraid to beat on the car to break that exhaust it in
