No direct experience, but will share what I have heard others say.
Many people prefer flat mounting for powder skis because it gives them more feel for the snow.? Most Rail systems have a certain amount (20mm or so) of stack height built in; this helps a lot with adding leverage in tipping the wide ski on groomed surfaces, but also detracts some from the snow feel.? Also, rail systems usually have a floating heel or toe (or both) so that the DIN pressure stays more constant and so that the boot itself doesn't stiffen the ski as it flexes.? Unless the demo mount had some sort of rail or floating heel piece, you've already found that you like the flex of the ski with (basically) a flat mount.
Summary.? The bird in the hand.? You know that you like the feel of the ski with a 'flat mount', assuming that this was how the demo ski was mounted.? You really aren't planning on using the ski on groomed surfaces that much, so the leverage of the lifter doesn't seem important, I would go with the flat mount.
Also, FWIW, the whole purpose of most of the "plate systems" is to free the boot/binding from stiffening the ski and they are designed and mounted such that they float on the ski and don't change the flex of the ski (very much), so I doubt that the Fluid plate is stiffening the 8000, but that's based on my understanding of plates in general and not the Fluid.? ?