Like almost everything else in this part of the hobby, there are many schools of thought. Some say AT LEAST 30ppm, some say AT MOST 30ppm. In reality it's very unlikely you'll have 30ppm at the point your calculations show it to be. Personally, I tend to err to the high side of the scale. It won't hurt to have 45ppm but plants won't benefit. Your fish won't suffer in the least.
To directly answer your question it's a common notion that 30ppm is the highest level of CO2
plants can benefit from. Someone else may know of a particular specie that can use higher levels but it will likely be a "weed" that uses it to take over your tank.