There is an issue with the CAT converter on 2001-2002 models but you may be too late with your mileage to have it covered by GM. See link below.
Click here
Your Regal LS should be just as fast as your Grand Prix GT or SE with SII 3800 engine with the exception of dead stop starts since the Grand Prix has lower final drive ratio gearing.
You can run an exhaust backpressure test to see if the converter is clogged.