Max real world internet speed for 8x4 cable modem

Hello,  I have a Zoom 5341J 8x4 DOCSIS 3.0 modem and am on the Blast! Pro+  300 tier.    Speed tests show on average approx 260 -275mbps so I was wondering if this modem which supposedly can support up to 343Mbps is at its max capability on the Xfinity network or if there is something that needs to done with a re-provisioning of the device?

Thanks for any info on this subject.