Hi Guys and BWD,
Line length can change the performance of your kite by either changing power or changing upwind ability.
Short lines (20m) reduces the wind window radius by 20%. The circumference of the radii is also shortened making the effective distance from one side of the wind window to the other a shorter distance. This means the kite traveling at the same speed as in a larger wind window will take less time to get across the window thus staying in the power area for a shorter period of time. The time the kite spends at higher angles of attack is also decreased this reduces the power and makes it quicker to get to neutral.
Longer lines (30+m) increases the distance the kite travels from one side of the window to the other so it stays in the power area longer which does not increase the power, but does increase the amount of time the kite is producing the power whilst diving the kite. The effect is that the kite seemingly has more power by staying at higher angles of attack for slightly longer.
The lesser known fact is the effect of increase and decrease of drag made by the lines. If you imagine the lines at the kite end (of the bar/lines/kite combo), are all being pulled through the wind at a perpendicular angle and are thus producing a reasonably significant amount of drag of downwind pull.
In a long line scenario, this drag prevents the kite from reaching its lowest angle or closest position to the edge of the wind window. In other words the added line length (consider adding 10m extensions) you would effectively be adding 40 metres of perpendicular line drag out beyond the normal kite position. This drops the kite back and makes it sit deeper in the wind window. This makes the kite produce more power than at previous neutral position, as it is held at a higher angle of attack and never gets all the way to where it would normally sit in neutral. The negative of this is you lose upwind ability and tend to fight the kite to go upwind. Further but less significant is the reduced kite speed from the drag, so although you lose kite speed you gain in angle of attack duration.
With shorter lines (20m) the reduced drag from losing 20m of line at the kite end of the bar/lines/kite combo, makes the kite faster and sit higher in the wind window. The kite will require more wind but be much more responsive and zippier.
Final word on long lines is you can go too far. More than about 40m lines becomes pointless for two reasons, the kite sits far too deep in the window and what gains you make in power you lose in upwind advantage and you'll spend the whole time edging the kite to go upwind and wash of speed, thus cancelling out any apparent wind speed gains. Longer lines beyond 40m (steering lines specifically) also create so much drag that they create long slack curves in the distance between the kite and bar and start to work like dampeners in the steering. In laymans terms, your steering gets all slow and spongey and can get to the point of not working at all.
Some kites respond better than others to longer or shorter lines and much of this depends on their profile and amount of inherent drag in their design. SOme kites may get very prone to stall if they sit deep in the wind window to begin with, and others that sit high upwind, may become less stable and more prone to luffing.
You should have a play around with lines, you can learn a lot and even reduce your quiver if you have two bars of different line lengths running on the same kite. I used to run 20m and 30m a long time ago and the difference is amazing.
DM (Sorry for the long winded explanation, I wasn't going to bother, but then thought, OWTF)
) always the case, what DM said. 
