First of all, you said a catback system, which means you didn't change the catalytic converter. If you did install a perf cat, then of course that will be different then what I said. In my comment I specifically said the cat is important to change.
Secondly I'd have to see the Dyno spec sheet before I fully believed that only a catback system AND a new catalytic converter gave 30hp, although it is now believable. I guess though you probably meant a new cold air intake and better ignition too right?
I have a 2003 Honda Civic Si... which has the 160HP 2.0L DOHC i-VTEC engine (code K20A3). I added an HKS Hi-Power exhaust system and can tell you for a fact that it didn't add much power. Unless your car came stock with one of those 1-inch diameter 80's style exhaust pipes, no exhaust is going to give your car an extra 30hp. I also have to add that your car comes stock with 127hp, not 130. If you added a cold air intake, header, high-flow cat, and catback system to your car you would be lucky to get up to 150hp (a more reasonable number would most likely be closer to 140...maybe 145). My Si (along with any K20-based car) can get up to 20hp from a cold air intake (Due to the very restrictive nature of the stock airbox. It has been proven that a K&N filter will actually cause a loss of 1hp with a stock engine.), but a complete exhaust system from the header back will only add maybe 5hp. My HKS exhaust with the stock header (yes, the Si comes stock with a 4-1 header) and stock cat adds 1-2hp and roughly 1 lb of torque. When I had my 2002 Mustang GT (4.6L SOHC V-8), I was going to put Flowmasters on it. I was only going to do it to make it a bit louder since I found that, even on an engine of that size, it would only add approximately 5hp.