but if some scene does not upload due to a hard limit, the time invested for reducing / merging texture maps would outweight the render time difference.

This really depends on the types of scenes you are rendering.  Some (generally landscape) scenes I've tested have 4mil polys, but < 10 texturemaps, and I can squeeze those scenes into my 1Gig graphics card.  Other (generally architectural) scenes may have only 1mil polys, but 300-400 texturemaps, and it's this type of scene where the 600 series has the benefit.  If you do 1 or 2 human figure renders without a lot of scenery/props/clothing, I doubt you'll hit the texturemap limit of the 500 series cards (although figures do vary widely in the number of texturemaps they use).

Paul

Creator of PoserPhysics
Creator of OctaneRender for Poser
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