Is it correct that when going down a bank and you brake (not heavily) the revs will go up to 3000-3500 and the engine will sound as if you've gone from 5th gear to 2nd gear in one fell swoop.

My wife's Yeti does that so I daresay all modern VAG cars with DSGs do so as well. The car detects when road speed is increasing despite the application of brakes, so it shifts down a few gears to add engine braking.

The additional retarding effect is small and the sound of the engine forced into running at up to 4000 rpm irritates me, but it isn't my car so I say nothing. There may be a switch that disables this function, look in the handbook.