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!
But what I understand 313 and 316 cdi has exactly the same motor, only difference is the software. So from 129hp to 163hp should not be any problem för the engine.
My any concern is the automatic gearbox.