You've got to watch out for power cables when doing this, not just to avoid drilling into them, but also make sure the cat5/e/6 does not run parallel with the power cables otherwise the EMI could reduce the signal quality and thus speed. You need to be at 90 degrees to have as little interference as poss.
Can you get the floor boards up and see if it might help you route the cables better?
You could also use trunking to hide the cable if you decide to drop if through the floor->ceiling.