Wow you trying to build a fence to withstand a EF5 toronado? LOL
I would have done 8x8's as the end posts and 6x6 for the running posts. If you wanted to make it stronger, design it with more corner posts (corners make fences stronger with long runs). You can extend them by cutting "L" shapes into them and caraige bolt them together, save the saw dust to make your own wood fill so the cracks to blend the two together.
If you planing on the gates swinging away, the posts on the end that span the gate, I would have sunk them deeper and given them a bigger concrete footing and making them a little taller to distrubute the wieght of the gate. To fix sagging, run steel cross members behind the gate forming a "X".
But at 20ft span you might want to look into a steel frame rolling gate, like the chainlink gate but no chainlink, fasten you dogear or pickets to the frame. The steel frame will handle the wieght of wood better and stop sagging. Also this will be flush to the opening as well.
Edit: also if you want to make it taller, you can fasten a latice type of topper to the top of your fence, It'll kind of split it up but still allow air movenment instead of acting like a giant sail.