Gigablast speed very poor after new service install

Hello all!

I'm not getting advertised speeds with my new service plan...  some background.

Yesterday I signed up for new service at an address that already had existing service.  I selected the 'Gigablast' plan and had to pick up a new modem from the cox store as my Roomate (gentleman whom had existing service at my address) was required to turn his rented unit in upon terminating service.  The prior service was the 'family preferred' plan, or something like that, running at 150 mbps.  

So... upon arriving home I installed the new modem, walked through the setup menu and ran a speed test.  Connected directly (wirelessly) to the 'panoramic' modem (technicolor unit) I was seining download speeds at or below 300.  These speeds were not real consistent and the speed test curve was virtually a straight line, in that the speed started low and continued to climb until the test ended.  Based on the speed alone, I initiated a chat with Cox service.  

After a factory reset and multiple restarts / updates triggered from the Cox rep remotely, I'm seeing speeds between 500 and 650 but not consistently.  Additionally, I was able to obtain a more conventional speed test curve - spiking initially to 300 and leveling off for the balance of the test time by putting the Cox unit in bridge mode and connecting my Netgear R8500 router.  In quick summary, I went from speeds of under 100 to over 600 with whatever magic / cap adjustments the Cox rep wielded during our chat.  Improvement, yes.... however, I'm concerned the service rep was not competent simply following a decision tree to guide his/her account adjustments.  

Long story long, I'm presently seeing relatively consistent speeds at 300 on my MacBook Pro wirelessly through my Netgear Nighthawk R8500 router.  I'm certain the router can handle higher speeds and because during testing I was able to obtain several speeds over 600, I wonder if there are not still account settings or adjustments that need made...  Much appreciate any thoughts or recommendations prior to my tech arriving tomorrow, I really don't want to spend hours arguing about paying for a service call when it is an adjustment that should have been made during my initial tech support chat.