Drilled and slotted rotors are not going to specifically improve your braking performance by themselves. The 84-87 cars used an 11.5" rotor (smallest of the C4 series) and is a bit thinner compared to the later C4's with 12" or even 13" rotors.
Your choice of brake pads will be a big factor. A good semi-metallic pad like the Hawk HPS pad will make a difference in braking performance. Raybestos also makse some good semi-metallic pads as does Performance Friction, and NAPA.
Semi-metallic pads will generate more dust compared to a stock-yype or ceramic pad. Unfortunately there are trade offs between a clean wheel and increased level of braking performance. But if you wash the car every few days, it's not really a problem.
Sometimes a bad brake power booster can be the cause of reduce braking; it's also possible that your pads are worn to the point where it takes more pedal travel to get the brakes to work.
Drilled and slotted rotors may look good but if you do any sort of extreme braking like you would encounter during track days or carving out the turns in the mountains, stay away from the cheapie drilled/slotted rotors. These versions have the holes drilled in them after casting and that effectively weakens the metal around the holes. Frequent heat/cooling cycles of these rotors will cause cracking around the holes and that can lead to rotor failure.
I autocross my 87 a lot and also do occasional track days. I usually buy generic "white box" rotors and just toss them when they have worn out.