I really dont think shortening your stem by 2cm will do anything so severe that you cant safely control your bike on descents.
The reach of drop bars varies by a good bit and the effective placement of the brake levers can be 2cm difference just based on which drop bars are used.

I have found the bars I use, Origin8 Gary OS Sweep, are comfortable going down hills as the drops are wide for stability. But I can also ride them on the hoods, tops, ramps, and drops in total comfort during flat road.