Have to ask about the obvious things: Was there magride recalibrated after the drop? Alignment done by a good shop? All suspension components checked for wear? If not, start there before you throw money at more parts.
Magnetic ride calibration will help a little but won't give you the result you desire, MSS kit/lowering springs/standard suspension all have this issue at high speed where the back end is uneasy, especially on a stage 3 car. The 034 motorsports rear sway bar does help a little with this but not enough I feel. I would highly recommend looking into a coilover kit!
Recalibration is necessary when lowering a car with mag ride. Get it done and your ride quality and handling will improve. If your car is tramlining at speed your alignment may not be optimal for a lowered car. Factory alignment specs aren't always the way to go, but it is what most alignment shops will of course aim for. If your car is particularly low you may need adjustable sway bar links, as these all be cockeyed on a low car and not working as they should. 034 motorsports makes adjustable links for our cars.