3 ways to increase the output while staying N/A:
Freer flowing exhaust. Getting rid of the dirty air is vital to getting in clean air.
Freer flowing intake. Once it can exhale, help it inhale.
More fuel. Once it's breathing better, you can start adding more fuel. This is a mod that radiply starts having diminishing returns though. As the fuel is increased, the power can start going up pretty easy, but adding too much fuel just makes more smoke and heat. Too much heat, and things can go very wrong.
Adding a turbo is as close to "free" horsepower as you can get. It pushes a lot more air in, and as the air is increased, less additional is fuel is needed to make the same power. In many cases, adding a turbo not only increases the overall power output, but also tends to increase the MPG. It also has a funny way of reducing the exhaust temps, since the fuel can burn better during the power cycle, rather than still trying to finish burning as it exits the cyl.
By design, a diesel wants as much air as it can get, at all times. The power varies from how much fuel you spray into that air under high pressure. When adding a turbo, it is not mandatory to turn up the fuel to see more power, but with the increased airflow, it can effiently burn more fuel, and create less exhaust heat at the same time, so it's standard practice to turn the fuel rate up. Just turning up the amount of fuel that *can* be injected, does not mean you will then *need* to do so.
Buying a turbo kit that is made specifically for the 6.9/7.3 isn't really required. Any number of turbo's can be used, with proper care and fabrication. Buying the kit for a 6.9/7.3 is just so much quicker and easier than a DYI install. Getting a good used kit is generally cheaper than most DYI installs, as long as the kit is all or mostly there.
New turbo kits are a bit pricey, but all the design and fabrication work has already been done, so it will fit like it was made to, as it was made to fit. Plus, you have to take into consideration that you are getting a new turbo, and they aren't exactly cheap by themselves.