300ps on only chiptuning is impossible as Jenni has said. The stock components in the SRT4 will not allow that kind of power.
The stock turbo is capable of 300whp+. Thats not the issue. The stock wastegate will not stay closed long enough to get you to the PSI you need to get 300ps. You would have to change that trim in the ECU to get it to work.
Next problem is fuel. The stock injectors running at 110% of cycle aren't enough to get you 300ps. Even a Stage 2 kit direct from Mopar gives you new fuel injectors and sensors so that you can get 260ps.
The final problem is exhaust. The stock SRT has 2 choke points. The small o2 housing and the exhaust rear section. A nice 3" exhaust will free you a lot of ponies.
900 Euro for the "idea" that you can get 300ps would be a waste of money. 900 Euro is about what... $1300? For $1300 you could get a good Staged computer to get your timing and fuel running nicely. A nice wastegate to up the boost. A good exhaust... I just watched a 3" side exit sell for $310 plus shipping... And even leave enough for an intake to get you some nice cool air into the turbo. For a few hundred more you can add a fuel pump and some injectors and really be talking about some good numbers.
Talk to any good tuner, Darrel Cox, AaronNeon, TK Motorsports... they will all tell you its a mixture of components and tune. There is no way around it..... you MUST replace components to get to the point where a tune is going to help you.