Nevin,
AFAIK the Impreza and Legacy do not use the coil on plug design the SVX does. They use a single coil pack and spark-plug wires.
I had the good fortune
to replace all 6 on my '93 and it made a huge difference. Examining the old ones I found that the wiring insulation at the pack was broken down, and apparently the cause of some spark leakage.
OH! The engine in my new '97 runs smooth as butter!!
Glenn