The newer Buick Regal GS is not considered a performance car in any way shape or form.
The 3.8 supercharged motor is a great engine and was the basis for the last great Buick muscle car, the Grand National (although the GN was turbocharged, not supercharged).
The supercharged 3800 engine can be found in some Pontiacs ( more of a performance brand) and the Buick V6 block is one of the most popular and most widely used in GM history. It is identical to the 350 CU IN small block V8 with two less cylinders. It would make a great starting pointo for modifications. But I'd look for a Grand Prix, Monte Carlo, Impala or even a Camaro or Firebird to put it in, not the four door Regal.
If you want a real Buick sleeper look for a 1986 Regal T-Type. It is the exact same car as the Grand National (fasted American production car of it's time) but it came in colors other than black (GNs only came in black). The only way you'd know it had the turbo and intercooled V6 under the hood was the slight bubble in the hood.