Light output increase is hard to estimate or to measure. I have read something about 50% more.
In my case, only switching the LED and not the optics, the output changes dramatically because the Seoul LEDs have a different height than the standard Luxeons. Originally the lamp produces a very bright spot in the center with some circular spill of moderate brightness around it. With the Seoul LED and stock optics I am getting a pretty homogeneous illumination of a wider angle than before. Overall much more spill but still good enough to ride fast in the night on the road with it as a single lamp. I prefer this type of illumination - if you would want a similar type than the original (obviously stronger) you might need to try the lenses discussed in the candlepowerforums thread.
The important thing is that the runtime of the lights does not decrease. The new LEDs are just 5 or 6 dollars now at kaidomain or dealextreme.