We gotta build a foundation for you to build on. There may be some misconceptions and misunderstandings that are throwing a wrench in your hard-use full-send max output plan. So this will be the redneck way....a lumen scientist will probably disagree but they won't win in a duel so animal kingdom rules win.
Think of light as energy being used/consumed, with heat being the unused energy.
Energy is supplied by the battery. The brighter an LED the more energy is being consumed. The more energy being consumed the more heat being generated. Heat eventually kills all LEDs...Malkoff lights have more conservative outputs for durability, reliability and longevity.
The theoretical brightness of an LED can be calculated and is given in all LED datasheets. There's nothing magical about it... if you send a certain amount of current (mah) to the LED the LED will respond with X lumens. The larger the LED footprint the more lumens and more flood the beam will be. The smaller the LED the less bright but the more throwy the beam will be. I'll post to the M91T datasheet below.
There's no such thing as optimal voltage. There's just a voltage range where you go from working to frying
There's a minimum amount of voltage it takes to light up the LED, there's a minimum amount of voltage it takes to "turn on" the driver.
There's a maximum amount of voltage an LED can take before frying, there's a maximum amount of voltage the driver can handle before frying. I can touch an LED with an ohm meter and power up the LED, or I can touch an LED with a 3v CR123 battery and make it as bright as the sun.
Elzetta and Malkoff LEDs are 3v LEDs. That means only 3 volts go to the LED, regardless how many batteries or what the voltage is.
So a 9-14v driver simply takes multiple batteries and limits the voltage to 3v at the LED. More than 14v and the resister on the driver fries.
So there's not really optimal per se....you match the output to what you need and for how long.
The formula we use is: Watts = Volts x mah
From here we can calculate estimated runtime based on how many volts we're using.
Think of Volts as light intensity and current as the rate of flow of the intensity....how long it can be sustained.
Volts "pushe" the current.
The less volts we have the more current we need, the more current we have, the less volts we need. They work together.
Malkoff drivers are regulated -- at least most if not all -- so that means within a voltage range the brightness will be the same. Eventually as the batteries grow weaker the regulated output goes out of regulation and the light just simple dims as you use it until there's not enough power to "turn on" the driver. At that point the driver won't send signal to the LED. This is just with Malkoff and Elzetta. There's many other driver designs.
Here's a great explanation of the eyes detecting brightness by Henry at HDS
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Malkoff's M91T is around 1250mah...so you can see how the light shines at an estimated 750 lumens.
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M61Hot will serve you well.