Not sure I can see how anything but flow would be a problem. As for bacteria, they will establish themselves at the proper level and simply maintain it. You could have too LITTLE media for bacteria, but you cannot have too MUCH, unless by that you mean more than they need to colonize.
In other words, I am not sure "overfiltration" means anything. The correct amount will always have a little spare capacity. You can have too much flow, depending on inhabitants.
Some plants like flow, some don't, so that could explain l8nite's situation.