I did both on my megaflow. First I took a dremil and lowered all the teeth. Then, I realized that the slits don't really affect the water height, at least not like I needed to lower it. It is actually the piece of plastic behind it that affects the level. I only ended up cutting the teeth on one side, but that side defiantly drains faster. Although my cutting of the back piece of plastic was anything but accurate and even between the two sides so that could be accounting for the difference.