So put 3 leds in series, increase supply voltage and set current to 350mA. Same light output (more or less) as one led @ 1A.
Or use a different led driver.
Btw, you have a timing problem if you want to make pulses of the short duration you're planning for. Datasheet of the driver, page 7:
The switching time is in the order of 100 nanoseconds
Oops. So 62.5ns is out of the question, and anything below 10x the switching period I would consider sketchy to begin with. So for all intents and purposes I wouldn't consider these led drivers for pulses shorter than 1us since they are likely to be unpredictable in terms of regulation (especially if you expect a 100ns pulse to be exactly half as 'bright' as a 200ns pulse etc.). Not to mention that the startup behavior isn't really specified, so you have no way of knowing how these drivers handle such a situation.
I think you're better off devising a linear regulation scheme for this from discrete components. It'll be somewhat tricky though given your <100ns requirement. Even N-channel MOSFETs can become somewhat problematic in use if you need consistent switching speeds in that region (you need to think for a few minutes about an adequate gate driver).