IIRC (and i'm no expert), any cat back system can be put on any contour/mystique (SVT, SE, LS, it doens't matter) but the SVT, especially after the Y-pipe has much higher flow. I know for a fact that you can attach the SVT exuast on it for less than $200. The side skirts will fit, but the bumpers and other contour body kits will not work. One guy over in GB or somewhere put on the SVT body conversion kit (search pics and vids), and it looked bad ass, but we concluded that he had to have molded panels together using fiberglass or some other technique becuase the bumper from an SVT contour will not bolt onto the mystique front end. Other than slight body differences, the contour and mystique are the same car. The mystiques have a much better looking stock rear end and hood, and they have so many more accessories inside the car (doorlights, floorlights, rear dome lamp, power seats, wood trim, nice leather, traction control and other things i can't remember). with some performance tweaks, you can make many contours jealous of the mercury counterpart.
Silver 1998 Mystique LS (v6)...ATX

Removed Emblems, limo tint, painted calipers.
Premier P630 HU with 4 Pioneer TS-A6895 Speakers with 10' Sony sub (yeah i know, i'm cheap)
Commando 501-S Alarm