Robban,
When I line up my template, there is one slug in particular that will overlap the existing PES2 pockets the most, by perhaps 20% of the surface area. If you fill the pocket with Arctic Alumina, there should still be adequate heatsinking ability IMO.
Four IMS20 will fit in a 2C Mag no problem. They fit nice and snug. With Luxeons the reflectors sit just high enough to clear the slight chamfered edge just inside the head, next to where the lens meets. With the modded IMS, the reflectors sit 0.030" lower, just enough to touch the chamfer. A couple strokes with a file on the side of the IMS where they meet the head would be enough to give adequate clearance. The 3 legs will not only meet the heatsink face, but 2 can be aligned to touch the short wall surrounding the heatsink. Epoxying the legs will secure them quite well. If it were a single reflector light, I'd insist on a McR reflector as the IMS can have some artifacts. But with four, I think any artifacts will be washed out by the other emitters.
In short, the IMS build, mainly due to having the legs on the reflectors for centering and securing, is the way to go. Once I'm finished, I'll do some beamshots and lux readings with the 2 different reflector setups and see if there is any appreciable difference. But I'll guess that the additional cost and difficulty centering the McR20s is not worth the trouble.
I said I trimmed the legs and back of the IMS 0.030", but that's a generalization. Actually I found the overall height of the Seouls to be slightly less than a Luxeon, and wasn't sure if the recommended 0.030" adjustment was from the back of the reflector in contact with the emitter, or from the base of the slug. I ended up splitting the difference, and trimmed 0.035" from the legs. I use the legs as the locating surface, and trimmed the back of the reflector enough to clear the emitter surround.
FYI I measured my IMS to have an overall dia. of 0.802". Amazingly enough, I recalled enough geometry to figure that the center to center distance of opposing emitters should be 1.134" (reflectors just touching each other). My template is basically a circle that fits inside the PES2, with 4 holes in it where the emitters locate. I sized the holes larger than the actual emitters so that I can visually center them, and also any excess epoxy doesn't squeeze out onto the template.
As I mentioned, without modding the heatsink face, the dimensions are right so that a 2.84 mm thick UCL will seat the bezel and lens properly without much gap. If you were to shave the face of the heatsink to clean up the pockets (any shop should be able to do this easily), the reflectors would sit just a bit lower. I still think the bezel will seat properly, but at worst a thicker O-ring will do.
If you're planning on 2 X 18650, you'll need a deep tailcap (Modamag has some gold ones still available), or an extension ring (Mirageman). 2X 18500s will fit without any length issues, just a sleeve.
I'm partial to external pots, but a cool build would be to make a can that houses the converter board and (+) contact on the underside of the PES2. Do away with the stock switch, and use a tailcap switch like Modamag's new release. It would be too bright so a multi-level converter or some other means (2-stage tail?) of dropping the output for close-up use would be almost a necessity. You'd have to make sure the tube end was sanded and butt securely against the heatsink for the ground path, and a grounding screw for the converter/emitter somewhere on the heatsink would be necessary. If I ever finish these current builds, I might try this with either a single or multi-emitter mod.
