I have both foiled and not and it seems that on a drum with a brisket that size you should be fine just remember low and slow, I would be careful not to heat it up to fast. As I understand it what seems to happen is the slow cooking of the meat breaks down the bonds that cause brisket to be such a tough piece of meat so your problem if it isn't tender could more be that the internal temp isn't high enough as opposed to you actually drying it out. Now that being said this didn't seem to be true for that four pound brisket I did that came out somewhat the consistency of trying to eat leather...
WSM, UDS, BBQ Pro bullet smoker, weber performer, weber genesis silver, weber 18", weber 22", smokey joe, and a [COLOR=blue]Super accurate, deadly quick blue thermapen.[/COLOR]