46) I think they are trying to say that the take off mass is limited by the speed of the tyres, therefore the downhill slope wouldn't help because the tyres still wouldn't be able to go any quicker.
273) the regulations are that the clearway used cannot be more than 1.5 x take off run available. So even though there is 2000m of clearway, you can only use 1500m of it (3000 x 0.5)