The only 3D tool I know of which can import b3d files is Ultimate Unwrap. It is perfectly capable of splitting a mesh into separate pieces and exporting b3d files, but the lightmapping will probably have to be setup again in code or using B3D Tweak.
As for your noticing that TrisRendered stays the same if you don't split the mesh up, that's because frustrum culling in Blitz is per entity, not per polygon. So even if you are only looking at a tiny part of an entity, the entire entity is rendered.
As for your noticing that TrisRendered stays the same if you don't split the mesh up, that's because frustrum culling in Blitz is per entity, not per polygon. So even if you are only looking at a tiny part of an entity, the entire entity is rendered.