who has actual experience adding c02?

hi i am interested in adding co2 to my setup in order to increase the yield. as soon as i have a little bit more money i was intending to invest in a propane burner and raising level to 1500ppm. my plants are currently getting 3k lumens per ft^2. I have been told that adding co2 with light levels below 7500 lumens p/ft^2 is a waste and will do absouletly nothing. is this true? does anyone have first hand experience with this for comparison? thanks

co2 works wonders, even at this level. 25% increase no problem.