I've set up two tanks with MGOCPM, and I didn't notice any ammonia leaching from it.
I was doing a fishless cycle immediately after set up, so if it was leaching, it was a tiny amount and just blended in with the cycle.
I've heard the actual composition can vary somewhat from area to area, so maybe it's an issue with some batches and not others?
I think clay would probably be better then plastic, but I'm not sure why. If nothing else, you can get algae to grow on it, and attach moss to it.