I dont have Gigablast so excuse my ignorance but, isn't just fiber to the home and then copper on Coax once it hits the demarc point of your home? I ask because your existing coax cable was not designed or installed with anything except television in mind - analog TV at that in most cases. Do you know if your coax is RG6 or the older RG59 standard? RG6 has a larger copper core which means a larger pipe if you will. It is possible you have a poor SNR (Signal noise ratio) which could come from a variety of sources including too much power, not enough power, amplifiers on the line that help TV signal but hurt internet, etc.
Unless we, as customers, are prepared to contract out a specialist who is going to open the drywall up throughout our homes and run Cat 6 and new coax throughout, we cannot expect to get the "ideal lab tested results" that are advertised. When a telecom trenches to your home to bring you 2020 technology, but it is connecting to 1970s-1990s technology, you are only as strong as your weakest link.
