Yes and no.
The fatman will boost your battery voltage to drive the LED's. But it can only boost, it cannot buck.
Let me explain.
Your LED is a current operated device. Max current is 1A for a P4/ XR-E.
At the maximum voltage, the LED will drop it's maximum voltage.
Download the spec for the SSC P4 and take a look at page 16, forward voltage vs forward current, Ta=25deg C.
You'll see that at 1A, the vorward voltage will be 3.7V.
3*3.7 = 11.1V.
Your battery voltage is 10.5V, so the controller you are looking at will work.
The bottom end of the graph shows that at around 100mA, the LED will have around 2.75V across it.
3*2.75=8.25V.
But your battery is 10.5V, and the controller you have is not capable of reducing the output voltage below the input voltage, so you will see an output voltage of 10.5V, which according to the graph in the SSC spec will give about 600mA.
The upshot is, that with a fully charged battery, you will not be able to turn the brightness down (below 600mA) using that controller on its own.
If this is important to you ideally what you want is a
Boost converter with PWM. The fatman has a constant current regulator, which is altered by adjusting the pot. A PWM controller generally has a fixed output current, that you can switch on and off rapidly. By varying the on and off periods, you can control the brightness.
Alternatively, you could reconfigure your battery placing cells in parallel (be sure to balance them first), to keep the same capacity but lower the voltage. But using this method has disadvantages. Firstly, you'll still have a minimum brightness, it'll just be dimmer. Secondly, you might find that efficiency decreases.
Hope that helps.
PS,
No, I don't have any idea where you might get a PWM boost controller. I am using one, but I'm designing it myself, it's in early stages and it won't be in a form factor for modders to use.
