Agreed that default Slacker is/was 128Kbps. After I got my creds from Tesla and updated the Slacker website config for my account, I too detected a less feeble quality (noticeable when testing, for example, "Gunfight" by the Sick Puppies, or any classical piece with strings, even) and a longer download (bar filling) time.
Perhaps those who tested at 128Kbps can test again after confirming the 320Kbps setting.
The operational issue being that getting 320Kbps is connection quality-dependent... *and* you have to have made the aforementioned credentials/website-dependent change first.