Iceman,
As much as it pains me so, I'm going to have to agree with Jerricho.....more a physical reality than artificial.......There is actually nowhere any more taxiways could go. Even though we'll get more stands with T5, the taxiways weill actually be more restricted because of wingtip clearance of the A380.
I understand where you say that the capacity of LHR is capped artificially......yes, if we took a busy hour of 90 movements, and did that for every hour, we could get 2100 a day rather than 1350. Trouble is we (as a tower, but applies to our Approach guys at Terminal Control, and also London Centre, and the neighbouring centres also) don't have the staff even if all the noise restrictions went out the window......for example, we usually have maybe 10 or eleven controllers on shift during the peak hours of the day......overnight we have three.
Using 23 actually decreased the movement rate. I forecast that it will never ever be used as a runway again.