Instead of laying the pistons on their sides to stop water from flowing, put them facing down, so the piston's arm is against the floor right in front of the water. This will make it more space efficient. Also, try using up to 4 rows of wheat before placing a row of water for hydration. Farmland works from 4 blocks away from a water source, so don't forget that ability. Using a 9 wide by 8 deep plot will make sure that the wheat growth is at full potential while still allowing you to walk above the farmland blocks. Oh, and don't forget to add light sources so the wheat can grow at night.
This uses the methods stated above to allow for 8 blocks deep of wheat with 4 blocks wide per plot. In all honesty, if you wanted to switch sides, you could get rid of that middle bar and just have an 8x8 plot and access all of the blocks from walking on the glowstone on the sides. Since you can reach 4 blocks out, it's possible to do this. Here is your most efficient wheat farm, though.
EDIT:
Image of the pistons up close