i am trying to use mesh collider with highpoly mesh in the scene. i think it will speed up things a bit. so i tried to make lower poly from the same object and then to use it as a mesh collider. but no luck, does it need to be in the scene with renderer disabled in order to work?
The quick answer: Yes :-)
Create an empty game object, attach it to your highpoly mesh object, attach your lowpoly mesh, add a mesh collider and disable the renderer.
You can get more performance when you combine primitive colliders instead of using your (lowpoly) mesh.