Max lumen output would depend upon what path you choose to take with respect to the driver. A flupic can push 1.2 A on burst through the LED (emitter), and make over 240 lumens according to the Seoul spec sheet. In reality, I wouldn't hope for that much. But you won't want to leave the LED running at 1.2 A for long because of heat issues. So, 200 lumens is the upper limit for what you may be able to get at 1 A (the maximum current according to Seoul) from the emitter, then factor in losses from the reflector and lens - you might get 150 lumens. Output from the Cree would be similar if you could get your hands on a Q bin, but even the P4 bin would be a considerable increase in overall lumens - just don't expect much more throw with the Cree since a lot of the extra lumens go into the spill.
While it would be best to mount the emitter (bare LED) directly to a copper heat sink, this involves considerably more effort than just purchasing the LED emitter already mounted to a star. The star is practically a direct swap for the Q3 star, but you will have to modify the reflector by filing up to 0.030" off of the bottom for best focus. The stock reflector is nice, but will not focus the Seoul to a super tight spot. If you want better throw with the Seoul then you should also consider a McR-20 reflector. Try the stock first, it is a useful blend of throw and flood with a smooth transition into the spot. Moving the emitter down towards the LED also means you will need a thicker O-ring to keep the lens tight (or two O-rings).
If you just swap the emitter and mod the reflector you will not be near 150 lumens because the stock board only regulates to 700 mA IIRC (maybe 750mA), and that is only with a CR123, not a RCR123. The higher voltage from a RCR123 puts the LED in direct drive, and while that was fine for Luxeon LEDs, you would not want to do that with the lower Vf Cree or Seoul LEDs because they would be driven way over spec. So, to get the most lumens you should also upgrade the driver board, and then you will also be safe using rechargeables.
Links for the
flupic, the
Seoul P4 U bin star, and the
McR-20 reflector.
Here is a link to Paul6ppca's Q3 Cree mod.
Here is a link to where myk modded his 301x-3 with a Seoul, though there is little detail.