I'm not in favor of using a treadmill for speed improving. It is so soft andd so the energy of your steps are absorbed by the belt itself.
You are an experimented trail ultra-runner and thus all your body is set for the long resistance trail running but, I am sure you can get a faster pace if you dedicate the training to do so...
You have developed an extraordinary aerobical condition, which means that your muscles are super-efficient managing the oxigen you breath at low heart frequency. Your race economy is excellent.
In order to run faster, you have to show your muscles how to work in a higher condition not only aerobically but also under anaerobical conditions, which means that your cells burns as well other kind of combustible than the sugars and fats...as a product of that reaction you get lactate acid which causes fatigue...the accumulation of which does not allow you to reach long distances...The training makes that your body treats those residual products much more efficient and so you can gain speed while running longer...
By the end of the running season I used to prepare always a couple of trail races and I can tell you that, as a result of the training on hills, my speed decrease a little but I feel much stronger in general. I feel that Trail running is much more complete than running on flat or almost flat runs. It is more complete.
YES, nothing is impossible and HARD is doable (I like this statement for @runningproject motivation quotes ;-) )