Now i have a little problem with tuning a Cessna 182 RG. It aint realistic, but i wan't it to go a little faster.
I was thinking about 280 knots cruise/ 320 knots max.
The plane's real-world cruise speed is about 120 knots, but you want it to go just a [glb]LITTLE[/glb] faster.

If so, get ready for a lot of investing in many things.
1. Understanding the dynamics files [Aircraft.cfg file and the *.air file]
You have to know what each value in the aircraft.cfg file means and you have to know how a change in one value can drasticaly change the effects of another value. I know this from a lot of experience.
Since my expertise lies only with ground-vehicle dynamics, my advice will be limited.
Since you wish to increase the speed of a Cessna to 280 knots, try reducing the gear-reduction ratio. The lower the value, the higher the prop speed becomes with less thruttle input needed. However, this might cause an increase in the response time due the Moment Of Inertia [MOA] exerted by the props.
Speaking of MOA, go to the Prop section in the Aircraft.cfg file and reduce the MOA value just a little. This might help with reducing the response time when you input the throttle. However, this might cause the some negative effects such as the engine reving up to the point that the aircraft won't sit still even with idle throttle.
Increase the HP might not do any good. I tried that many times and it still comes to the same conclusion: Over reving, Plane can't hold still, powerfull torque effects, etc.
This takes me to one last thing: Torque. This is one thing you gotta watch out for. When I decreased the Gear-Reduction on my Enzo Ferrari, it caused my model to either pull too much, too often on one side, or bad torque effects [the visual model might spin out when it leaves the ground - ex:when my Ferrari fell off a cliff].
As you can see, tunning the dynamics files will always cause problems one way or the other. Sometimes, the problems are unexplainable.
Other than that, I can't help any further. The rest is up the those who deal with aircraft development.