None. First off, why do the same job twice---get the springs and the dampers at the same time, or at least do the job at the same time. Second, it's well established that the stock dampers don't actually do their job very well. So adding a lowering spring that essentially is stiffer than oem will ask the shocks to do even more more work that they already aren't very good at.
If you look up the Best Drivers Under $35k issue of car and driver for a year or two ago, they published damping curves for all of the car, including a BBK mustang GT. It was vastly worse than every other car being tested---miata, GTI, 370z, Evo, and a couple of others. You wouldn't notice a change in ride height, but your handling would probably improve if you just replaced the dampers with something better.
But yeah, do it all at once if you can.