1) Around 315, i would go higher but this BS already noticeably affects performance during sieges. I didnt tweak reinforcement waves.
2) Absolutely, default BS deosnt go well with PoP's party sizes.
3)No, but imo it would be pretty easy to adjust renown gain with increased BS/waves so i dont see any problems.
4) Aside from high BS leading to damn npc's getting stuck in weird places during sieges, no.
Edit: typos.