I've not seen any data on slightly high magnesium levels. I do discuss issues with very, very high levels in this article:
Magnesium in Reef Aquaria
http://www.advancedaquarist.com/issues/oct2003/chem.htm
"Toxicity of Elevated Magnesium
There have been very few studies on the toxicity of elevated magnesium on most marine organisms. Most toxicity studies involving magnesium use freshwater species. This is largely true because magnesium is already quite high in concentration in normal seawater, so to significantly elevate it requires conditions that would rarely be encountered in oceans or even lagoons.
Bingman pointed out in a previous article that at elevated concentrations (>8,000 ppm), magnesium has been used as an aid in shucking oysters, helping to force the oyster open, and also as an anesthetic for them. Consequently, magnesium does have potentially negative biological effects at significantly elevated concentrations."