TLDR: figure about half the wattage for LEDs as for fluorescents.
LEDs are better aimed
Fluorescents (and other discharge lighting such as HID) are already as lumen-efficient as LEDs. What makes LEDs more efficient is aiming.
What you want..... what you get without at least reflectors.
So you need 8000lm to land 500lm on the path. (and mind you, we are thinking in 2 dimensions. In 3 dimensions, the math is worse.)
Here's what reflectors do (coarsely).
You get 200lm of direct light and about 300lm from the relatively inefficient reflectors. So in this case you might get it done with 3200lm of light.
(the laser guys can get highly efficient reflectors, but they must keep them fastidiously clean. In the real world, up on a pole, fuggedaboutit.)
Note that so far, we are presuming all light sources emit light in a circle (a sphere, actually; this makes the numbers worse.) That is true for every light emitter ever made so far - candles, incandescent/halogen, fluorescent, neon and the other arc-discharge lights. When lights are rated for lumen output, they are placed in a spherical room with sensors in all directions. Therefore the lamp's lumen rating includes a whole bunch of lumens you can't use!
However, LEDs are a different deal. LEDs naturally emit in a cone of 120-160 degrees. That's still more than you want in most cases. But this lends itself to a much better method than reflectors. Optical lenses. These are close to 100% efficient.
Because of this, LEDs can let you plant almost all of their lumens where you want it. Actually sourcing a light with perfect lensing can be a trick, though. So you may have to settle for something "in the neighborhood".
Anyway, when you're replacing fluorescents with LEDs, that seems to work out to about 50% of the wattage. Because we're not improving the efficiency, we're just improving the aiming - and typically not by that much. The fluorescent-replacement "LEDs" generally throw in LED's default 140-180 degree spread, just as fluorescent reflectors do.