I have had better luck in the mountains by slightly using the brake peddle to slow the car for curves in the road.

The brake mode seems to me it's for really steep grades and is designed more to keep the car about 35 miles per hour or slower on the descent.

I have found when the car is going downhill at about 65 miles per hour the manual brakes don't slow the car as well as at slower speeds. My guess is that they can't use the regeneration as well at the higher speeds. Could be its using the engine and some physical braking to slow the car instead.