You will only lose power if your exhaust piping is to large for the amount of power your engine produces. Oversized piping hurts exhaust velocity, since exhaust pulses are too small and no longer exit in a striaght mannor. Instead they bounce off the inside of the pipes, thus slowing down.

Like this



A properly sized exhaust system for your engine output (or future output) will not cause perticular low end problems.
Make sure your collector, cat, and exhaust piping fit your overall goals for engine performance.

It will add HP, because you have a bottleneck at your cat right now. Quality Carsound cats flow almost as well as a test pipe, and it will be 2.25" (or bigger depending on your exhaust diamater) instead of 2".