For what it's worth, I used to use LED, and lost 2 clams after a couple of months. Now I use 2 x 250w halides, and my clam has a massive white growth ring and is growing like crazy. He probably added at least an inch or 2 in total size over the last year. I had him in my old setup under some smaller halides, and he did ok, but now he is wide open all the time, and the mantle even touches the substrate sometimes. I want to add a maxima. Those are amazing.
In my experience, it's all about lighting with clams. The more, the better.