Even with a thin fat cap, it can make a big difference in moisture content when it's finished smoking. Fat caps are our friend when cooking something with a low inter-muscular fat content...The brisket point is just the opposite...lots of inter-muscular fat and very forgiving to cook.
Personally, I like doing a full packer because of having some of each type...and burnt ends from the point are to-die-for as appetizers. And, that massive fat cap holds in so much moisture when smoking...the only drawback with a fat cap is smoke cannot penetrate the fat, but you get a good smoke on one side of the meat anyway.
Let us know how that flat came out, anyway. I for one, am interested. I have done alot of point/flat separations before the smoke and a few of the flats were a tad on the dry side...these were very lean trimmed with virtually no fat cap though.
Good luck!
Eric
Edit: I just read back and had a thought about the drum...it seems that alot of folks go fat cap down to protect the meat from temp swings and maybe this helps to keep it at a more even internal temp. The drum shouldn't have much in the way of hot/cold spots directly in the center of the grate, I would think, anyway.
Maybe some drum-heads with more brisket experience will be along soon and have some info to offer.