When I signed up for Business HSI two years ago, they said D3 would be rolled-out sometime later that year, and I'd get an automatic bump from 6/1 to 12/2. Didn't happen... until sometime early-to-mid last year, but I did get the bump--without having to change modems. (Tho I had to call and ask "Hey, what's up?")
12/2 is good enough for our uses.
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.