I don't buy the heeling argument
First, the probability of having another boat close enough to see the spectacle while underway is minimal, unless the boat is near shore where discharging is illegal.Second, the deeper the thru-hull the greater the column of water you need to pump against when emptying the head, independent of its location. The head location above the waterline only affects (reduces) the pumping effort while pumping liquid but helps little while pumping air during emptying.Third, the deeper the thru-hull, the faster the boat will sink if it malfunctions.OTOH, the location of the intake thru-hull may present a problem while underway because it may suck air if the boat is heeled in the wrong direction.