I think it depends in part on your riding style, and how risk adverse you are.

As Webster points out it's not a question of stopping the bike, its a question of the rims over heating.

If you descend fast, using the brakes only to slow for turns, not to limit speed overall (i.e. braking hard but sparingly) you're less likely to overheat the rims, than if you use the brakes to limit your speed down the descent in general, and are on them pretty much constantly.

So slower descending teams actually need to be more concerned about rim over heating.

Personally, I would try the bike out as is (perhaps with new pads) and do some of the steeper descents you're going to ride, stopping frequently to check the rims, and see how it goes. That will tell you whether you want to add a drum.

Our experience FWIW

http://everestchallengex2.blogspot.com/