Fredrik,
I have a 2004 T1N 313 CDi Manual with a large coachbuilt body on it, 3 m high and 6.5 m long, and run at around 3,650 to 3,700 kg and find the 129 HP engine fine.
I would not want to risk the reliability of the engine by chipping it. I believe the only way to gain more power from this engine is by increasing fuel rail pressure and then risking injector damage or over fuelling.
If I where you I would leave it as standard until you have driven it for a while. And 138 km/h is 85 mph and I would not want to drive a Sprinter that fast for long!
Keith.