Just did a Stage 1 on my 435d.
Here are the results:
HP: 305hp -> 361hp (320awHP)
Torque: 616Nm -> 709Nm.
I can definitely feel a difference between the original and the tuned version.
The accelerations are more aggresive and it could be just me, but it feels like there's less lag when accelerating.
Anyway, I haven't been able to test it thoroughly as the shop told me to take it easy for the first 500km (aka not to do launch controls, etc).