You know, I have blamed high Co2 for years seeing plants in my tank like AR or simple Java moss not growing, and then was just because I was dosing too much traces... Just to say, sometimes you may just blame the wrong thing, it can really be that simple.
In my experience and knowledge by reading a lot about it, Co2 is a damn beast to control correctly in our tanks. It seems easy, but it I not. Tom is right saying that Co2 is the most difficult part to master. In my experience, there are too many variables that can screw up Co2. That may be dissolution, distribution, degassing, plant mass, water movement, flow, flow direction, diffusion, timing, duration, pressure, needle valve, and more.
In in my case, I have almost rule out Co2 by giving it maximum dissolution (I.e. No bubbles in the tank), 1.4 pH drop, consistency throughout the week and decent distribution in the tank. Also, plants are positioned in a way to have the chance to get Co2 wherever they are... Flow direction in my opinion is very important, you could inundate some plants of Co2 in an area whereas other plants could get 10 times less of it just a few inches away from that area... To many variables, for sure it is hard, but I don't think too much Co2 can cause harm in plants. If you consider how much more Co2 there is outside the tank, in the air, you should then be afraid if aquatic plants get out of water and start growing emersed... Surprisingly, they grow much, much better.