According to the data in Diana Walstad's book "Ecology of the Planted Aquarium" even the so-called "hard-water" plants should do more than fine with 3 ppm Ca and 1 ppm Mg. There's no physiological need for supplying more. The plants are not physiologically able to uptake more. Believe it or not, according to my experiments, plants under high light and high CO2 levels can uptake (consume) no more than 1 ppm Ca per week. Still, they can do fine even under much higher Ca/Mg concentrations (as is our experience), but that does not mean they have any further advantage from these elevated levels.
But you should be aware also, that too high Ca/Mg levels can work as "stoppers" that can block the uptake sites on plant leaves, thus blocking the entrance for other cations (mainly trace elements). So when you have high Ca/Mg levels in your water it may be wise to increase the trace elements dose a little, or use better chelating agents for iron (like Fe-DTPA).
PS: I don't know what EI people base their recommendations for such a high Ca/Mg concentrations on. They should explain it. Based on scientific data and my own experiments I doubt we need such a high levels, so I can't imagine any real-life scenario where there is any need to add more Ca or Mg into your tap water.