This is pretty cool. Culling stuff into the two hemispheres is obsolete here. Other than this the usual comparison between cube maps and dual-paraboloid maps applies:
- the number of drawcalls is the same ... so you do not save on this front
- you loose memory bandwidth with cube maps because in worst case you render everything into six maps that are probably bigger than 256x256 ... in reality you won't render six times and therefore have less drawcalls than dual-paraboloid maps
- the quality is much better for cube maps
- the speed difference is not that huge because dual paraboloid maps use things like texkill or alpha test to pick the right map and therefore rendering is pretty slow without Hierarchical Z.
I think both techniques are equivalent for environment maps .. for shadows you might prefer cube maps; if you want to save memory dual-paraboloid maps is the only way to go.
Update: just saw this article on dual-paraboloid shadow maps:
The basic idea is that you do the WorldSpace -> Paraboloid transformation in the pixel shader during your lighting pass. That avoids having the paraboloid co-ordinates interpolated incorrectly.