Hello again!
Climb rate has a large impact upon the operation of ATC. To my knowledge (feel free to correct en-route bods) climb rate is mainly an issue in the TMA environment. For each SID there are generally 2 plans a controller may initiate to solve route/level conflictions and for jet a/c, and climb rate is usually the deciding factor between them. Of course our job is to deal with whichever combination we're faced with but it is always easier to solve a situation if the a/c have good rates of climb or descent.
For those of you departing to the south of Heathrow the most complex single departure is the Gatwick Lambourne SID. A right bugger. Most of the regulars have some idea of it's problem, having to climb around OCK and BIG and requiring you to be at or above FL130 by the Heathrow centreline. For the shuttle type flights their climb rates of 3-4000ft/min+ make the task much easier than a lumbering A330 at MTOW!
Unfortunately just building a third runway doesn't do a great deal to solve the problem. It may help Heathrow Tower to a certain extent shifting a/c off the ground, but I'm sure it would create an even more complex situation for the Ground controller (a VERY difficult position), and another problem is dealing with the buggers once they're in the sky! At the moment Heathrow launch one almost every minute and an extra runway wouldn't, by itself, do much to increase this. Most parties further down the food chain complain about being delayed on the ground (not entirely without reason) but you have to understand, and agree, that it's a far safer environment than risking overcrowding in the skies. That's always got to be the priority.