You can set up the polyphony however you want - i.e., if a filter & amp are before the PolyToMono node, then they are polyphonic; anything after the PolyToMono node is monophonic. It's up to however you want to design it. If you have other topologies for example patches, please post them!
Polyphony doesn't seem to add to big a hit to CPU but I would like an option to limit it to certain voices. I think there's an example patch running polyphonic delays up here somewhere.