I have two S15's now, both from Gearbest, and although I'm in trouble with them on other orders, these came quickly enough. I promised a mod thread, but although today I had some time for the mod, I had not enough time for a full photographic tour of the modding process. So it will be description of how it went.
The bezel was quickly loose using a piece of rubber inner tire for grip. Then pushing a pencil into the battery tube I could push the (press-fit) pill out for some distance, but not fully: it gets stuck against something sticking out from the side switch. I left the pill at that position and managed to screw out the reflector by placing the piece of rubber tire over the reflector, and by pushing my needle-nose pliers on the edge of the reflector with the rubber in between slowly the reflector screwed out (I knew from my modded S20 that the reflector is screwed into the pill. I scratched the top edge of the reflector a bit but that goes under the silicon ring anyway. Then the led centering plastic was removed and the ledboard was cleared. Unsoldering the ledwires inside the cavity was not easy, I guess they used solder of higher than normal melting temperature. I was going to use an old 3500K 80CRI Luxeon Q that I still had (I tested the Luxeon Q two years ago) because I love the tint, and I reflowed it on a 16mm Noctigon because the stock board has XM-L sized pads. Before reflowing I sanded the Noctigon quite a bit thinner but not as thin as the stock board (for modding the second S15 I probably will not bother using a DTP-copper board because the current will never be over 1A in this light). I managed to solder the Noctigon into the cavity (blobs as far to the side as I could get them, to electrically clear the reflector) and placed the centering diagonally relative to the led to get a perfect centering. Now the reflector was screwed in again using the rubber piece. The centering ring rotated during this process, but with the reflector in place I could rotate it back by placing my tweezers in two opposite corners of the centering ring square hole and twist. Then the lens was placed back and the bezel screwed in. The rubber piece was used again to get the bezel real tight (and get the ledboard pressed firmly onto the pill, I suspect from how the blob of thermal paste looked under the stock board that in stock form the ledboard was not fully pushed down!)
The performance with the LuxeonQ compared to the stock XM-L2 went way down, even more than I hoped for (but I'm not surprised, with the smaller led and way warmer and higher CRI tint). But the light has now the lovely warm Luxeon tint, and with its smaller hotspot, judged by eye, it still throws noticably better than with the stock led (have not measured throw). Numbers after 30 seconds on high setting: stock light on 14500: 310lm, stock light on alkaline: 155lm, with LuxeonQ on 14500: 165lm, with LuxeonQ on alkaline: 105lm. The current on high setting with a 14500 was 880mA in both lights.
Some pictures. As you can see in the beamshot (left the modded light, right my other S15, still stock), the LuxeonQ has a bit more blue in the hotspot compared to the very cosy yellow corona, I noticed that too in my Lumintop Tool, that has the same led. I does not show in the beamshot, but the modded light does have more throw than the stock light. Looking with the camera into the reflector on moon setting shows that the led does not receive a completely constant current, some stripes are visible (interference with the camera, nothing you can see in real). Oh, I'm never good in keeping all dust out of my modded lights 
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