I would not recommend using thinning shears to reduce the density of over dense systems. It will leave a lot of short stubby hairs in between long hairs that can feel almost like a mohawk near the base...been there and done it! Thinning shears are very good for tapering down the system to reduce some bulk around the ears and lower back of your head where the systems hair meets your own.

The entire hair needs to be removed if the hair is too thick. If you don't want to feel a bunch of stubby hairs near the base or see little short hairs sticking straight up amongst your longer hair don't go thinning shear happy through your hair you'll regret it.