muffler

I have a 1990 buick lesabre limited sedan. I found a dynomax ultra flow ss muffler that will fit my car. however, i have heard that putting a performance muffler on a regular car cas decrease the backpressure and actually slow the car down. I have gotten several different opinions, and each person tells me something different. Some tell me it will slow me down, some tell me it will improve power, and some say it won't really do anything. if anyone out there is familiar with how exhaust systems effect engine performance i would greatly appreciate your help. Also, the muffler i am looking at is shorter than the stock muffler. i really want to get it because it has dual outlets and a dual exhaust was my goal, but i am unsure if a shorter muffler will have any performance effects or reduced sound quality. i want a deep, true performance sound. thanks

This should answer your questions about backpressure:
http://www.automotiveforums.com/vbulletin/t13199.html