Theorically, there not should be a great difference between using proxies or meshes. The amount of memory required for a specific geometry is constant, the difference is that using proxies the render engine can unload it dinamically, if the system if running low memory. In any case, Forest stores an unique copy of the object, because all the scattered items are instances of the source.
I know there was a severe performance hit in VRay, when the scattered items (technically the voxels) intersect between them. But this problem was discussed in the ChaosGroup forum, i don't know if they fixed this issue in the latest releases. However, this affects only to the items, not to the source geometry.
I think would be a different problem in your scene. If you like, send us a copy and we will check it.