Well, good luck with it. Typically the guys who brag about makin big power with a few bolt ons have more done than they are letting on. I mean little detail things, or doing things like removing accessory belts for dyno testing.
Also, you are currently somewhere between a US spec setup and a euro. The euro setups were only rated at 139hp at the crank. Granted you have a slightly larger engine and header, but you have a lousy injection system compared to the european version.
I know when I had a 16v in my Golf, the car ran ALOT harder once I yanked the CIS-E injection and installed a K jetronic system with lambda.
Im not saying you wont find a few more HP in doing details, but I doubt you are going to find 20hp without replacing something or doing some serious chip tuning.