The reason the slicks throw up more water IS because there is more surface area. This coupled with the grooves going along the circumfrence of the tires basically sucks up the water. The water then exits the grooves when it has reached a centripital acceleration great enough to release the water from the hold of the grooves and it just so happens that this is usually straight up onto the rider. Sooooo unless you're going to be doing any hardcore speed-turns, knobby tires will be fine. Just don't be too much of a daredevil unless you want to end up on your rear end sliding across the pavement! But that could be fun with like motor racing leathers on or something...