The key to stride length vs. frequency is based on the ability to open stride behind you not ahead of you (over striding). If you can hold a cadence of 180 strides per minute while increasing stride behind you, your speed will increase. If you open your stride ahead of you, expect to slow down due to vertical oscillation.
That being said: if you can increase stride by 6" behind you at 180, you will improve by more than a minute per mile. On the other hand, if you over stride by a mere 3" at the same cadence you will slow down by 30 sec. Per mile. The solution is to fix your running mechanics.