KDFW is a major problem in the game with the capacities as is. In reality it's one of the busiest airports on the planet and has, I believe, THE highest capacity (though it's not all used). It should be about 5 TIMES what it is in the game as its capable of over 250 movements (125 "slots" as defined in the game) per hour, a truly massive number that few major airports even come remotely close (80-120 movements is common, in excess of 150 is rare, and DFW sits at 250)
I've already sent Sami a list of all major US airports and the PROPER capacity for them as deemed by the FAA. For most major US airports they're off by a factor of 2, but that's because Sami's using generic data. He's going to update for the next game.
It's unlikely we'll ever see a game where ever change in every airport is taken into account though. So you won't see slots suddenly change due to an expansion. Right now they're based off a modern figure (I believe Sami was using 2002 data for most things, the FAA capacities I sent him were from 2004) and they're adjusted on a straight line from there. So they increase as time goes on, but only modestly.