When I was running around with just my BBK O/R X-pipe and no mufflers I have NO backpressure and unless you dumped it from 3,000 RPM's and hookedracing was not possible. The total lack of backpressure made the car unlikable to drive. The sound was baddass but fuel economy went way WAY WAYYYYYY down.
www.youtube.com/crainracing
When I got my Borla's on it upped my backpressure to acceptable levels giving me pleanty of torque and power up top.
I should also mention that it is illegal to run without cats in California. I have been checked in the past for cats, CHP doesnt f*ck around if they suspect anything.