I think the Drager is 85mm. This will attach to the back of your cat, or "test pipe" if you remove the cat. The Drager will be a lot bigger than the stock header and cat setup. You can get a turbo cat with 76mm openings vs the NA's 65, but even this is not as big as the Drager. The Drager will definitely be louder, but I am not certain it will result in much more power than an 65mm NA exhaust. I think the Drager will bolt on all right, because as I said the holes on the attachment flanges are elongated for just this contingency. You will need to center the gaskets: you may want to use O-ring copper, to get a good seal.
Contrary to popular misconception, you don't need any backpressure to run the engine; but if you put an exhaust on (everyone who is not running on a track has some kind of exhaust) you have to consider the flow of the gasses through the piping. There is a value known as the Reynolds number which applies to all fluids flowing in a pipe; I am familiar with this for water flow through sewers. Basically there is an optimum pipe size to minimize turbulance and maximize throughput. Because the calculation is cumbersome for exhausts with bends and different sizes, the best "advice" is don't do too much to disturb the flow. Consequently, I don't think you will get much HP advantage unless you increase the size of the entire exhaust, including tuning the headers. But it will be louder.
