I am still using last year's firmware, and on mine I am using "8", which is a reasonably fast spool up. Also there is another quantity which is throttle response, which takes over (according to CC) at about 80% of the throttle setting. I have that setup as the default value. It is really only important for that last 20% or if your throttle changes--like mine does on the 3rd lap (after takeoff) from 7600 rpm to 8000. I like a smooth change to the higher speed, not an abrupt one.
I am pretty happy with the spool up rate. It is fast enough so that if I dare try an unassisted takeoff, that the plane doesn't muddle around the circle at very low speed, but not so fast that it noses over either.