Extreme Pro 600Mb plan with Netgear CM600 only delivers 350Mb

I upgraded my xfinity internet to Extreme Pro - 600Mb download and I am only getting around 350Mb download speed.

I tested by connecting an ethernet cable directly from my macbook pro to the modem and ran xfinity speed test. ( no router, no wifi ). Other speed test site like https://projectstream.google.com/speedtest gives about the same results.

What could be the issue? Netgear CM600 is 24X8 and rated for 960Mb on xfinity's website.
I contacted xfinity and they sent some diagnostic signal and claimed it is all green and normal on their side. They suggest contact modem manufacture, but I doublt it will re-solve the issue.

My modem page shows all 24 download channels are bonded. 5 of the 8 upload channels are bonded.