Haven't tested in 10, so just a quick look in 11, and a lot going on in the scene. Some of the decals don't render for me (you can see what I get in the image attached).
That is a HUGE list of multipass render elements there, turning those all off left a render time of 24 seconds for 15 passes compared to 4 minutes. Are you sure it's not those that are causing the difference between 10 and 11 rather than the decals? If you turn those off, are 10 and 11 then the same even with decals on?
Also it has passes AND noise target set, meaning perhaps one end-condition is triggered differently with decals on/off/10/11. Usually best if "speed testing and comparing" to either used fixed passes, or fixed time, and to disable noise (it was definitely stopping on the noise condition for me in my first test).
Some of the geometry of the road looks a bit unhappy, as I am seeing some triangulation there in the rendering.
PS as suggested I just added a default sun sky and lowered the exposure.