Ok, I just bought the Borla cat back system from Summit (http://www.summitracing.com/parts/bor-14888) and don't have anything else done to the car yet and back pressure was a huge concern. Should I expect to lose power of gain and about how much of a loss or gain? Its 3" piping with the cut outs.

The car is a 91 Z28 with the 305 TPI. Approximately 140.8K origional miles, the exhaust had to be replaced do to a decent leak in the pipe behind the cat.

 

If anything you would gain power as the borla's flow really well. Make sure you have a nice high flow cat too, and I would go the extra mile and get a nice set of shorty headers too.