Back in the 1990's, kitty litter became "popular" in planted tanks, but like many of these fads [others I recall were Laterite and substrate heating cables] the benefits were minimal at best or non-existent so the fads petered out.
The only one I tried was laterite under the gravel in one tank. I saw no difference in plant growth over a period of several years.
Where all of these might benefit is in a completely high tech setup. As with any factor in a planted tank, be it light, nutrients, CO2, enriched substrates...there has to be a balance or plants simply cannot take advantage.
Which brings me to the Eco-Complete. I used the basically identical Flourite substrate in my 70g, and after more than 15 months I am very disappointed with the lack of benefits. Later this month I intend tearing this tank down and chucking this stuff, going with Quikrete Play Sand which I now have in 5 of my 7 tanks. The same plant species, under the same light specs, and with the same fish foods and plant fertilizers, are growing as good and even better in the sand tanks.