Gen,
It has been my experience for many years that neither position nor force gradient are particularly important for speed stability, since most helos are so dynamically unstable that the pilot is constantly correcting cyclic to keep the aircraft level. The most critical term for speed stability is the Theta fuselage vs speed slope, that is the nose down gradient with speed. The only helos that give me trim troubles are those where the nose is flat or negative with speed.
Often this attitude/speed gradient follows the stick gradient, but usually because the attitude is the dominant factor, and the stick follows along.
The horizontal tail is the bad actor, where rotor dowwash playing on and off it will make the nose pop up with a slight speed increase, bewildering the pilot and the static stability.
I have been a champion of flat stick gradients, as long as the pilot has the cue environment to trim with ease. The new draft 29.175 FAR/JAR words were pounded out in a subcommittee meeting with some of us reaching consensus that these other cues (nose gradient) might suffice.