You turn fastest with neutral throttle. One of the devs explained how it works. I may not be remembering this exactly, so don't quote me. When at neutral throttle, one engine has full thrust forward, the other backwards. When throttle is not neutral, the turning engines hold the same ratio to each other (up to max output), thus making your turn slower. The reason this is is because one turning will max out at full forward (since it can't go double thrust forward), while the other goes to neutral.
Neutral:
----->
0
<-----
Max forward (turning right):
---------->
----->
0
As you can see, the RED indicates the thrust that would be given by the left engine IF the thrust ratio was preserved. Since it can't go beyond max thrust, this is not possible and you basically get 50% turning power, while also losing a good deal of forward acceleration.
Going in reverse does allow you to turn faster than forward, since your main throttle is not maxed out. This allows less loss of the turning ratio. However, it is still a slower turn than neutral.