I was studying how the waterwheels derive power from the blocks around it, and concluded that it seems to only care about the blocks directly adjacent to it, or in other words, the diagonals don't seem to factor into the stress units calculation. This means that, if you manage to make water flow in a cross around the wheel's axis, it should be able to achieve maximum power. Using this knowledge and a few blocks of Soul Sand and Magma, I made this little monstrosity. It's a 5x5x5 power source, outputting 7,680su@20rpm.