As far as i know bryce's architecture was traditionally based on parametric geometry, which probably means it deals with terrains much differently then it would with a pure 4-million poly imported mesh. At least i've never heard of someone importing a mesh with hundreds of millions of polygons in bryce yet i have heard of people using hundreds of terrains and/or lattices, which should amount to hundreds of millions of poly's.
So though i havent tested this, my suspicion is that, unless bryce's internal architecture has been built specifically to cope with huge amounts of polygons, a collapsed mesh (which is essentially become a poly-mesh) might actually be less economic then an un-collapsed mesh.

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