I have a Sportster Replay and got that email last week. I went ahead and replied by getting the attenuator plug, which arrived last Friday. It's so small I'll be lucky if I don't lose it, with the different kits I'd have to use it for (2 car kits, and 1 home kit). Obviously, it's a 1/8-inch jack, but the plug is only about 3/4-inch long.
Whether I use the plug or not, I doubt I will. But I figured if I ordered it it would help Sirius look good to the FCC in regards to the compliance numbers. Starting September 1, and every 3 months after that, they have to report to the FCC on their compliance status. And if the numbers aren't what the FCC is expecting, it's hard saying what further action might have to be taken. Of course, in regards to that, a person has to consider that the merged company is telling people with non-compliant radios that if they don't use the FM feature that they don't have to do anything. So, maybe when the FCC looks at the numbers, they may figure that in when they go to compare how many non-compliant radios are out there with how many of those are now compliant, and then also compare it to how many complaints they're still getting.
You have to love, though, how Sirius goes about trying to get people to go along with this, by saying in the email that by doing this it'll improve the sound of your radio.....when that is NOT the reason to have the subscriber do it.