Download speed dropped when switching from Motorola SB5101 (Docsis 2.0) to SBG6580 (Docsis 3.0)

I have the Premier internet package which is advertised as 50Mbps in my area.  I ran speed test using speedtest.net and the cox speed test prior to swapping out my new modem and was getting 18-24Mbps down and 3-4 Mbps up.  After switching to the new modem I'm only getting 9-9.5 Mbps down and 6-8 Mbps up.  I tried calling in and they re-pushed the config but I experienced no change.  I contacted Arris support for the new modem and they had me list off my downstream power and SNR and upstream power but everything was in an acceptable range.  The Arris rep also said it sounded like an issue with the config file being used. Anyone else had an issue like this with this modem?  Any suggestions?  This modem was on the approved modem list so sending it back is going to be very frustrating.