I have not weighed the splitter I made. It actually wasn't even a splitter, it was just a chassis mounted undertray. Mainly made it for 2 reasons:
A: Test the chassis mounting
B: protect my oil pan and bumper from getting destroyed because I used to do street driving.
That being said, I street drive it maybe 2% of the time, very seldom. The race group/class I compete in restricts aftermarket aero, so I will be removing my chassis mount and maybe saving it for the future if I bump up a class. You may want to look into a racing league (ie. Gridlife, NASA, Global Time Attack) and build your car to a specific class spec (if you haven't already).
Regarding the chassis mounting hardware, I purchased the battle aero mounting brackets. They are OK but under heavy load, I do think the two mounting points are not enough and cause flex in the splitter. I would make 4 additional mounting points on a splitter with the Battle Aero chassis mounting hardware (2 in the middle, and 1 on each far side).
See image of their product:
