12/2 doesn't require D3. I have a D2 modem and I have Charter's 15/3 package with SpeedBoost/PowerBoost/[whatever]Boost. It peaks at just over 30Mbps for the first few seconds and then slowly crawls back to the high teens.
Under optimal conditions, D2 modems can do 38/27. However, depending on plant design/capacity/management, they might require D3 modems even though your package's speeds don't reach the D2 limits.
