I remember in the past with my porsche 911, I got a horsepower gain by removing the catalytic converter and replacing it with the stock straight pipe from germany to get some extra hp. Does this apply to the GS400's?

 

It applies to any car just about but you have to make sure you don't ru lean from doing it which I suspect you might. Also, without certain precautions you will trigger the CE light. Lastly, the GS400 has more than one cat so it will take quite a bit of fabrication to eliminate all of them.