Decat-ing your stock exhaust will improve your power. It would be like having a very short 4-1 manifold. However, as you did not specify just what car you are driving, hard to say whether removing the cat would cause an MIL for the engine to light up. Most likely the oxygen sensor would indicate lean mixture in the exhaust. If you have more budget, I'd suggest changing the entire extractor to either a 4-2-1 or 4-1. Having used both, I'd say they both improve power. If you love revving the engine as I do, a 4-1 is better and the torque peaks at high RPMs. If however you wish to tune your car for better pick up, go with a 4-2-1. Either way, you won't regret.