There are a combination of things that are at play that determine whether a flashlight will start fires:
1. Bulb position compared to the front of the lens. A 20W bulb in a small D26 style reflector is enough to start fires or at least a lot of smoldering (just takes awhile). A portion of the heat energy that comes out the front is directly radiated off the bulb, proximity to the bulb plays a big factor in how hot it gets.
2. Reflector size. A huge amount of wattage in a very small reflector, will put all of that thermal energy into a very small surface area directly in front of the light, making fire-starting easier.
3. Reflector type and quality: The glass reflector of a track lighting module, is actually designed to reflect primary visible light forward, while allowing thermal energy to escape through the reflector, to be absorbed by the housing it's installed in. Whereas well made aluminum reflectors with good finishes will reflect a LOT of thermal energy forward, (which is a bonus as it keeps the flashlight running as cool as possible given the circumstances)
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I have found that I can get paper to smolder with a the ROP high, takes awhile, but it will get a decent amount of smoke rolling, it's pretty hard to actually get it to ignite though, have to get just the right amount of air-flow across it.
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For easy fire starting with a mag mod, I'd say that an aluminum reflector with smallest bulb opening possible, starting around 60W things should get pretty good. (ROP is around 30W give or take). 100W+ and it'll start fires in seconds....
However, with a glass reflector, (if you were doing a mod involving a track lighting bulb), it takes over 100W to start fires reliably, and it still takes some patience as so much of that heat goes out the back.
but then if you move up to larger reflector sizes, it takes more watts to start fires, big turbo-style-reflectors don't start fires as easily from what I have seen.
