I bought a P-tronic tuning box for my 320d. It should arrive in 2 or 3 days.
They say that the power will increase from 177 bhp to 226 bhp, while the torque will go up from 350 to about 460 Nm.
(to be honest, i wish that the power/torque increase isn't going to be that great because I'm afraid for the clutch, gear box etc. - 215 bhp with 420 is ok by me)
I'll give you details, after i install the box.