Have you powered off and on your cable modem? And is it docsis3. You have to do both to see if you have the new speeds and no powerboost.
Theoretically 38mbps, but they probably won't let you get anywhere near that on a D2 modem, as you'd be saturating a whole channel.
What's the speed limit if it's not a Docsis 3 modem? It would be great if they told us this. So I could have more than 2X my speed but need to upgrade the modem?
Everything above performance requries a D3 modem. If you have blast and not getting the speeds, call and make sure the modem is provisioned with the correct bootfile. Speak to tech support ONLY!