Having read the Wikipedia entry I'm not convinced of its useful application in combat:http://en.wikipedia.org/wiki/Khat

It shows the effects as:
Khat consumption induces mild euphoria and excitement, similar to that conferred by strong coffee. Khat can induce manic behaviors and hyperactivity similar in effects to those produced by amphetamine.
There are also unpleasant drawbacks too when suffering from withdrawal.

I have a vague recollection that in Somalia, in 'The Blackhawk Down' incident, a comment that if the raiders had gone in later most of the local fighters would be in non state to fight - having had their Khat chewing session.