A large part of your instability at speed is the suspension. As any cop...in the US....which he would rather drive at speed. A Camero or Mustang? The Camero, in police trim, is much more stable at speed in part due to the wider stance and better suspension.
One trick the budget racers use is an air dam that extends from the lower front bumper to as close to the ground as you dare. Suspension travel and curbs/bumps come in to play here if it's a daily driver.
Look at some of the old classic muscle cars. Many came with a nose spoiler. Most of these were not effective, but they did force some air out and around the sides which is the goal.
The various bumper and skirt treatments do two things. They only allow air to the radiator and to the front brakes and possibly an inter-cooler. The rest you want routed up or around the car.
The object is to get as little air traveling under the car as possible. At speed that creates a low pressure zone and this has a slight suction that helps hold the car to the ground.
As far as rear deck spoilers go unless you have access to a wind tunnel most are just dead weight! Too many variables in a street car for a one fits all approach even if we are only talking Mustangs! The ride height, the front body treatment, the stiffness of the suspension and even air pressure all effect the angle of attack and thus the design of a rear spoiler. There is not a one fits all solution.
Hope this helps you rethink your at speed dilemma.
Body kit to help car more stable at speed 
