The Diamond X700 series is the highest gain dual band ham antenna ever made but its expensive at about $400 shipped. It would be a little better than the X510 and a slightly noticeable but not huge difference in performance. Harvest makes a copy of it, also 24ft long and its $225 shipped. In my experience the off brands like Tram and Harvest are not that bad and a lot less $$. You would never want to put any of these on a mountain top repeater but for home use in mild weather they should last awhile.
Its really hard to compare them on paper because the gain figures are all made up. The 700 is about 20% longer than the 510 so 1dB sounds about right. It also depends on where the gain lobe lands, if its at the horizon on one and up or down on the other then the differences between them will be greater.
Their gain figures are probably predictions and assumptions based on how many elements are stacked.
But the radiation pattern diagrams looks almost as if they where measured, with side loobs and all. Of course they show that the main loob are perfectly angled at the horizon. Diamond® Antenna ~ X500HA/X500HNA Radiation Pattern