Here's a topic I see all the time on here.
Something we need to consider are the laws of basic physics - specifically, energy.
Let me use a perfect example that I have first hand experience with. (2008 models) Inova T2 MP vs Inova XO.
Both use the same two CR123 cells, both have the exact same reflector design, same size head and bodies that are very close in design. The only differences in these lights is the LED itself, the knurling around the light and the tailcap. T2 uses a clicky switch, the XO is a twisty with momentary pushbutton.
So physically, these lights are very similar.
The XO will run for around two hours before really going dim. T2-MP will run much longer while producing more light while producing less heat. Its emitter is much better at converting energy into light, so this is why it has lower current draw and produces less heat while making more light.
The point is, one set of batteries has a finite amount of energy stored in them. Your flashlight converts this energy to light energy. Sure you can get 400 lumens out of a small carry light, but your batteries may only last 15 minutes or less.
There will be a point at which maximum efficiency is reached. This is where your light turns *all* of the energy it pulls from the cells into light and produces no heat.
So a 400 lument AAA cell light that produces 500 lumens and lasts 12 hours? I'd call that impossible.
However, we are seeing some awesome products as of late. I am very impressed with what I've seen and look forward to the advancements still to come.