Holy something that smells bad. I didn't see the price when I posted the first time. You are way better off getting a Seoul P4 or P7 or a Cree MC-E or XP-E. For the price of one of those things you could get a quad die LED (P7 or MC-E) that will put out about 30x more light. Don't bother with that thing.
If you want to modify the maglite to LED, I'd suggest trying to find the right a drop-in first. If you can find it then look into modding the light. Look here.
I'd ignore the 5mm and 10mm LEDs. Most of them have similar output. The 10mm ones just have a narrower beam angle. Highpower LEDs can be quite a bit brighter then the 5mm LEDs. On average it might take 40 or more good 5mm/10mm LEDs to equal the output of a single current gen high power LED.
I tried a 15k mcd 3mm led and it really isn't much brighter than the stock krypton bulb.
I want to try a high powered led but I'm not sure how it works and I also can't find one that would fit my flashlight. I saw a lot of mods and they always involve placing the led onto a pcb. What's it for?