The G25 initializes itself by performing a quite noisy full 900° rotation everytime you plug it in. Is there any way to remove this calibration ? Does the G25 really need to do it every time ?

Saves you calibrating it yourself every time, look at it that way.

Quote from Dissident :

Is there any way to remove this calibration ?
Does the G25 really need to do it every time ?

No
Yes

The G25 actually doesn't send any steering data if it doesn't do its calibration first. I guess this is the Logitech way of making sure the "help, my wheel is pointing sideways when driving straight" tech calls are kept at a minimum.