Superimposed preeclampsia only means that you had preexisting hypertension (BP rose prior to 20 weeks of pregnancy). It isn't a descriptor of severity. Superimposed PE could be either mild or severe, according to the regular criteria (BP over 160/100, 24 hour protein over 5,000, I think there are some others).
My PE was superimposed, but also severe.
Superimposed preeclampsia is a warning for future pregnancies, because chronic hypertension is such a risk factor (of course, so is previous PE of any stripe). I've not found good statistics on the likelihood of recurrence in that specific scenario, but my OB was very cautious.
Aliza - 01/05/2007 - Severe preeclampsia, emergency CS 37 weeks
Isaac - 09/26/2011 - controlled on 150mg Toprol, NO PE, 39 weeks!