First of all, you won't get any more power, you will actually lose quite a bit. But you will get a lot of noise if that is what you want. You will need to go with much richer jetting.
I don't recommend straight pipes. This is not a lecture about loud pipes. The reason I don't recommend them is that they have almost no back pressure, and and the engine and intake system is designed to run with back pressure. The big probllem comes from the fact that they do have some backpressure, but it will vary widely over the engines rpm range. Since proper air/fuel mixture requires fairly consistent back pressure, which even loud aftermarket pipes have, there is no way to properly jet the carbs. Carbs cannot be tuned over the range of back pressures that open pipes have at different engine speeds. The result will be that you will be way to rich at some speeds, and way to lean at others. This is a fairly efficient way to kill an engine.
Many Harley riders used open pipes anyway, even knowing what it would do to their engine. When fuel injection came along, they first hated it, because it was not traditional. They quickly learned to love it however, when they found that it, unlike a carb, could be tuned to provide a proper mixture over the entire rpm range, while still running open pipes.