David, most tail servos are very fast. With the new attitude controller in 3.4 and later I like to get rid of the feed forward and use settings like this. This is from a 500 with a Futaba 9254.
ATC_RAT_YAW_D , 0.007
ATC_RAT_YAW_FILT , 25
ATC_RAT_YAW_I , 0.28
ATC_RAT_YAW_ILMI , 0.0
ATC_RAT_YAW_IMAX , 0.3
ATC_RAT_YAW_P , 0.3
ATC_RAT_YAW_VFF , 0.0
Basically speed up the rate PID loop by setting the low pass filter to 25Hz. Use more P gain (and usually quite a bit of D gain). If you get a very rapid oscillation of the tail you may have to reduce the D gain a bit from what I use.
Since the tail servos are so fast I shorten the linkage a little bit too for UAV use by moving the linkage ball on the servo arm in closer to the servo shaft.
With the old attitude controller in 3.2 and 3.3 it used to take some rate feedforward on the tail, and the filter was usually run at 4Hz by most people with heli’s to get them really stable with no tail twitching. It took a lot of experimenting to find out setting that filter higher and increasing the P gain seems to steady them out.