Recent Upgrade to 300 Mbps plan yields the same speeds

I haven't thought about it or had the time to post about it since the upgrade, because I thought maybe there would be a difference over time.  I previously had a plan that advertised 150 down and 15 up.  I got speeds that were sometimes at or above these and was very satisfied, but recently I upgraded my plan to 300 down 30 up because of a price reduction, and I am getting 30 up, but still around 150 down.  The tests done before and after were both done on ookla's speed test connected to the cox - New England servers.  Is there something I need to update?  Could it be a hardware limitation?  I receive the same results using the router-based speedtest and a wired computer with a CAT6 Ethernet cable.

Any help is appreciated.