I see.
I used what I found in 'How do I create smooth 360 degree movement on a controller?' post for the angle, but I don't really get the distance part.
Right now the player smoothly rotate 360 degrees but how should I tell him to move depending on stick impulse ?
EDIT :
I might have found a not so bad solution.
I attached a bullet behavior to my player rather than an 8 direction. On top of the 360 direction I spoke of earlier, I'm setting the bullet speed relative to which ever gamepad axis value is higher than the other.
It might not be optimal but it does the trick.
Thanks a lot for your help !