Paying for Gigablast, only getting 220-270 Mbps

Hello,

I have gigablast in my community, I have had it since November.  The best I have ever gotten is 400 or so.  Typically I get 220-270 Mbps.  I have tried multiple routers, tweaking the routers and no luck.  I have spoken to tech support and had an on-site visit, but no change in my speed.  This is very furstrating.  I am aware that Cox does not "support" wifi, but that is what most people use.  ANny suggestions from anyone?

Thank you,

millerpt