I think it depends more on the flashlight's circuitry than the cells.
Some lights will be dimmer with a NiMH than with an alkaline because they have lower voltage (1.2 vs 1.5). Lithiums will be brighter still.
Some will have identical (or close enough to it) output with either alkalines or NiMHs. These have regulation circuits that boost the lower voltage.
In either case the NiMH will still give better overall performance. An alkaline simply can't pump the juice like a NiMH can, nor does it sustain its voltage as well.
Thus, even in a flashlight where the initial brightness is higher with an alkaline, after a bit of time, the alkaline will be dimmer while the NiMH is still going strong. For example, with the full 3 volts your stock Minimag bulb will burn brighter on fresh alkalines, but this doesn't last. With NiMHs it might not burn as brightly at first, but it'll still be going strong when you're at the point with your alkalines where you're looking for another set of batteries.
With flashlights with boost circuits, using NiMHs will not only give you the same output, runtime will be greatly increased. Take a look at the runtimes for the SL Propoly Lux or Fenix L2P.