96 mustang GT with pi swap needs help on what's next

it's not what supercharger is best for the car, but rather what is best for what you want.

Centrifugal builds power as rpms climb, and really plays to the strengths of an overhead cam engine. They move more air the faster they spin, feeling kind of like a turbo as they give little gain off idle but big gains at high rpms.

Positive-displacement types, like Roots and twin-srew, build power across the powerband. Basically increasing displacement, as they push the same amount of air every rpm and give roughly the same boost in power regardless of rpm.

Both are nice in their own way, with advantages and disadvantages to each. In simple terms, a centri's high rpm boost is safer as there is little time for detonation. And you get a huge rush of power as rpms climb. On the other hand, the broader power band from a positive displacement blower results in a faster 1/4 mile car for a given peak power level because there is more power everywhere.

That's just a gross simplification, ignoring things like added weight, heat soak, ease of using intercoolers, etc. etc. I don't think either one is bad, but whether it is ideal depends on intended use and personal preference.

If you still have a stock bottom end your compression ratio will be around 10.7:1 with the PI swap. So unless you want to run VERY low psi I would not get a power adder. Or you can kiss your bottom end good bye.