You get more PAR with water in the tank than without water. The air to water interface slightly focuses the light, because off center rays are refracted a bit towards the center. And, with water in the tank the glass to air interface at the front, back and ends of the tank reflects a lot of light back into the tank, increasing the PAR near the glass.
Many of the aquatic plants we grow are found in partial to full shade in nature, so they are not adapted to grow in full sunlight - 1500-2000 micromols of PAR. I haven't found a good reference for the PAR in open shade or full shade, but it is probably around 500 micromols. So, if you want to use the maximum light that might be used by emersed plants, you could try for something around 200 to 500 micromols. That would be extremely high light for an aquarium.
You can get a lot of light, without spending a lot, by using 23-25 watt CFL screw-in bulbs in dome shaped reflectors less than 15 inches from the plants. I think that is what I would try if I wanted fast emersed growth.