Plates from Kirk, Really Right Stuff, and WImberly generally work with the clamps on bullheads from all of the bullhead manufacturers. The question might be whether you would be happier with a custom place, one specifically designed for the bottom of the D800.
The L-bracket is for convenience. With the plate, to shoot vertically you have to tilt the camera so that the stem of the ball drops into the slot and then reorient the view. And with the stem in the slot, you can no longer level the camera by ball adjustment. Also, you moved the lens a few inches to the side, which changes its perspective to the subject, and for close subject may you to reposition the tripod.
With the L-bracket, al you do is loosen the clam, spin the camera 90º so the other side of the L is in the clamp, and relock. You still have full bullhead control and the lens is essentially in the same position ti was when shooting horizontally.
Bottom line: for my D3s I have both; for my D800 I have the L and may or may not buy the plate — for horizontal-only shoots I prefer not to have the extra L section.
Jon Kandel A New York City Nikonian and Team Member Please visit my website and critique the images!