What is a better gearing combination for downhill?
I ride a hard tail over all sorts of terrain. Generally I ride on the middle chain ring in these situations. Its less to do with the mechanics, and more to do with whats possibly coming up, and fast + reliable gear selection. When on the large chain ring, you can get caught out by a short (or long), steep uphill, and not be able to get a low enough gear to climb it just changing the rear cluster. 
I reserve the large one for easy predicable ground, or tracks I know really well - for instance sealed/gravel roads, manufactured fast tracks with sweeping / bermed corners where speed is all important, and you are unlikely / have no need to slow down below about 20km/h.

If the track is steep and rough, I am unlikely to run out of cadence before 'balls' - if you find you (like many others) have a "lower center of gravity" than me (i.e. smaller brain and/or bigger balls), and you run out of cadence, then go for the top chain ring. Also the chain can be dropped far more easily off the top chain ring - although on a well tuned bike this should not happen. 

As far as chain slap and derailleur bounce - I ignore it. Its a mountain bike designed to be used and abused. If you want, you can get neoprene coves for the chain stay which will quieten it down, or wrap it in duck tape... I have never heard of derailleur damage caused by slap, but that is not to say it does not happen.