It's not an awesome exploit but i want to share it.
1)Run straight ahead
2)Use démonic leap
3)You have approximately 0.5sec to press jump
If you do it right, you'll see that your character will jump faster and longer that a normal use of it.
I did some test but sometimes it doesn't works. Just train yourself to do it.
(i didn't tested with a buff that increase the speed, maybe it could add speed to the demonic leap)