[masraum]

The difference in the '84-86 US cars and the '87-89 US cars is a slightly different mapping in the DME and also that Porsche just was conservative on the initial hp claims. They did the same thing with the SC that went from 180-188hp with no changes.
Reading BA's book and the section you describe, the book isn't saying that Porsche added a cat and got another 10 hp. In the US they had a cat before and after the difference. "...wring a little more power out of the catalytic converter version of the engine," just means that they were able to take the US version that came with a catalytic converter and more highly tune it to get a few more hp out of the engine, not adding or removing parts. If you read the rest of the paragraph it says that the Australian version detuned the car to be able to use lower octane fuel which is why the car was back down to 207hp.
Basically, Porsche designed the fuel and spark curves in the software for the '84-86 version conservatively because there is no knock sensor and so no way to prevent detonation if they were too aggresive with the igntion curves. When after some testing and realizing that the US has fuel of a high enough octane and quality they realized they could go more aggresive on the fuel and ignition mapping. That's where the extra hp comes from. But when they did that they thought that you couldn't get the same high octane, high quality fuel in Australia so they basically left the fuel and ignition mappings the same so there would be no danger of detonation with the lower quality fuel. All Carrera's in the US from '84-89 had cats. Removing them would probably, at least, change the power curve and probably increase it at some range. There are high flow metal cats that you can get that are better than the stock, but they are pricey.