I've done some very successful mods of side-emitting Luxeon headtorches (Ring Cybalite Extremes) with Seoul P4s.
I removed the Luxeon emitters from the stars in-situ (which turned out to be very quick and easy) and thermal-epoxied the P4s in their place.
The reflectors did need opening up at the back to allow the P4 body to poke through. On the headtorches I had, that was just a slice with a Dremel and cutting disc, since the reflector base wasn't part of its support, and could be chopped off without destabilising anything.
The focal position was slightly out - the P4 needed to move a mm or so forwards relative to the reflector.
In the cybalites, the housing behind the reflector was cone shaped, with the reflector resting on three small radial ribs standing proud of the housing. To get good focusing just involved shaving off most of the thickness of the three ribs, and then placing a few small blobs of epoxy on the reflector front edge. When set firm, those blobs meant the reflector was held back solidly in its new position.
I guess I could have moved the P4 forwards by epoxying a small aluminium shim behind it, rather than moving the reflector backwards, but moving the reflector was very little effort.
Compared to the original setup with the Luxeon, at the same power setting, the spot was brighter, wider and smoother, with soft edges rather than bright/dark fringing rings.
As far as spill is concerned, the P4 gave more/better spill running at 25% power than the original light did at 100%.
The overall result was a light that was greatly improved, with about 3x the light output at a given power level than previously.
No real increase in runtime, but with a light that was more usable on low settings, it was certainly possible to make cells last longer by using lower power levels.