No, the 96 Seville STS was not equipped with a factory supercharger. I don't believe there are any aftermarket s/c setups either. The Northstar V8 is not very mod-friendly.
With that being said, I think the Northstar V8 would provide sufficient power (roughly 300 hp + 300 ft-lb torque) without forced induction to satisfy you and just about everyone else.
Have experience with Northstar V8's, as I previously owned a 2000 Cadillac Deville.