I have one of the Cree Dracos, on burst I get about 22 minutes before I can really tell the light is diming drastically. If you need more run time on at that light output level, get the extension tube. Just be prepared for how warm the light will get.
I agree that using a lower level will get you more run time with plenty of light out the front.
I don't have one of the new Q5s in mine but I do plan to upgrade soon.
Rob