And here is a Bryce render of the same tree. Even at regular render settings it took almost as long to render( I stopped it partway through the last anti-alias pass) as it did to import the tree .obj. As you can see, it is possible to export trees with leaves from Carrara, but wheter or not it will be worth it, or usable is entirely up to you. According to Bryce the converted mesh weighs in at 1,423,330 polys, but Bryce tends to turn every thing into Tri's when it converts an imported mesh. I would try exporting some of the tree presets as they aren't likely to be as heavy when you export them. HTH  Dan