What's your suggestion for soil simulation by physics?

I'm working on a loader simulation project, I can deform terrain as I want and for simulating loaded soil in bucket using small primitive object (sphere or cube) and when they hit terrain converting them to terrain's height map but as PhysX is not multi-threaded, when object's count reaching 250 ~ 300 or above, FPS drops and cause lag.
I have tested some solutions like; separating objects into two collision groups, decreased solver iteration count, using rigidbody.Sleep() on awake, using object pool to avoid instantiation cost, etc.
My last solution is using another physics engine like BEPUphysics, uPhysicsPro, to reduce pressure from main thread and use all system resources but it takes a long time to rewrite most of the core features of application.

Can you suggest any solution to improve performance?
Is there any multi-threaded physics engine that is compatible with PhysX?!


My system spec. :

CPU: Intel Core i7 3770K @ 3.50GHz
RAM: 16.0GB Dual-Channel DDR3 @ 666MHz
VGA: 2048MB NVIDIA Quadro K2000
OS: Win7-64bit
Unity3D Pro ( version 4.5.3f3)​