2017 performance pack Sachs
 

What is the current thinking as to how much of a drop on the 2017 Sachs dampers can take and still give good body control and decent life expectancy?

What is your definition of decent life expectancy?

To add to @swarb's question above define the kind of life you mean? Tight and twisty with metre deep potholes every 15 seconds or a billiard table smooth freeway with a 2mm ripple strip every 1/2 hour.

How long is a piece of string?

RCE says the RCE Yellow springs work well with the PP Sachs shocks. A couple of members are doing this swap. Then when the shocks wear out replace with Bilstein B6.

The more important question to ask is whether the 2017 shocks are suitable for the spring rate you will need to effect the drop you have in mind.

Assuming you don't mess around with bump stops that are too short the dampers don't care where they operate within their normal stroke. Shocks are usually pretty tolerant of operating in different ranges of stroke.

You have five inches of ground clearance. Dropping by more than 1.5 in would be ill advised. This would not affect the shocks.

Increasing the spring rate may well take your suspension outside the designed operating range of the Sachs from a spring damping perspective.

Assuming that the dampers aren't still over damped for stock spring rates.

Correct. Which begs the question of how far off the factory choice of spring rate to damper rate is.

In my judgment based on my setup I would say the 2017 spring rates are very well matched to the Bilstein B6 have. I would be very surprised if Subaru selected stiffer damping rates for the Sachs package than the Bilstein B6.

If I am correct then one would be foolish to fit higher rate springs to the Sachs damper set.

Which is my point. The op is asking the wrong question.