The biggest difference "performance fuel additives" make, is to the wallets of those making and selling them!
If you want some fun out of a Mirage, why not try getting better mpg by learning and adapting a different driving style? I get a long-term "B" average of 3.9 to 4.3L /100km depending on seasonal temperature, which comes to well beyond 55 MPG. Thats what I call fun!
Considering your from Germany, with BMW and Mercedes being such giants of industry, do you ever face ridicule for having a Mirage in your country? Snide comments, jokes, teasing, etc?
You can maintain your car's existing power by using an additive such as Techron to keep things clean. You won't however increase power. A good fuel cleaner such as Techron simply cleans the fuel system to work the way it should.