The stock brightness is going to be impressive at first, but it will rapidly fall off thanks to the increasing internal resistance in alkaline batteries. Stock runtime will be practically forever at vastly reduced output.
The Terralux modules have a checkered reputation around here... many have and like them. However, there have been numerous reports of driver problems, lousy LED tint, and overheating problems. This last is to be expected, since the Terralux dropin isn't heatsinked properly. To make a long story short, I've heard enough bad stories that I don't want one. Still, YMMV... They
are sold at
http://www.batteryjunction.com, and MattK, the owner, doesn't usually sell junk.
The best drop-in mag-mod out there (IMHO) is made by Gene Malkoff, one of the mad scientists here at the forum. His webpage can be found
here. Gene's dropins ARE heatsinked properly, which means that the emitter can be driven at maximum capacity (read: as bright as possible) without damaging it. Unfortunately, Gene's dropins only work in up to 4-cell mags, so your 6-cell would fry it.
If long runtime is your goal and you want to use the 6-cell mag but with decreased output (as in it will be bright for a few minutes and then decrease in output as it heats up), go with the Terralux. If you want to get more output with a slightly smaller mag (starts out brighter than the Terralux's maximum and stays there because it doesn't heat up) get the Malkoff. The Malkoff dropin will produce a reasonable amount of light for several days after it falls out of regulation.