I have a few thoughts on this...
The iPhone is very difficult to take apart. I repair computers, laptops, and some phones/MP3 players, and I have to send out iPhones/iPods to a specialist. They are entirely glued together, and require heating and prying to get apart. I wouldn't try unless you have some practice and all of the right tools for the job (and a replacement screen/digitiser for when that cracks).
The iPhone may not have the heat sinking ability to keep the built-in LED cool for long periods of time, as it's intended to be a flash. The app that locks it on may cause damage to it. A brighter LED would usually require more heat sinking, and there's likely not a good way to do it.
45lm is quite enough for a lot of tasks, especially from a back-up light, like a phone would be. If you need more light, there are plenty of lights that will put out more light, for longer time, are very small, and won't eat up your talk time.
If you understand the risks, it would be interesting to see. You'd have to see what kind of LED it has now though, so look for some teardown pics/video, or do it yourself. Then figure out if there is a more efficient version or bin of the LED in the same package. It would be very difficult to adapt a different kind of LED in something so small.
