MB8600 not getting Gigabit speed

I recently upgrade my plan from blast to gigabit. Purchased a new MB8600 modem. After activation checked speeds and I get speeds from 500-650 max. Tried testing also by removing router and wired from modem to computer and resulted in same speeds. I have power cycled, using other ethernet cables and even checking for un needed splitters on out door wiring. Only splitter i found was from cable outside coming from post then to a splitter that splits it to 2 lines 1 for the 1st floor customer and the second line to me. I have talk to comcast about this speed issue, all they do is just reset modem over and over. At the end they told me they will have to escalate my issue and that they will have someone call me in the next 24hr. Is there something else I should try?