"let's say I have particle set P1 and it's attached to O1. If you use copies of O1 in vis_contents of O2, O3, etc. you still only have the one particle set and only one set of particle calculations. However, in the renderer RenderParticles() gets called for each copy, so there's at least some amount of extra GPU usage. However if you render to a target you can get the same effect more cheaply on the GPU.
I think however building in a virtual render target for this sort of case wouldn't be a bad idea at all."