I decided to mix 3D with 2D. Most part of the conception process has been moved to Unity, which I treated as a canvas, where simple meshes are colored by "WIP" textures, image effects, lights, shaders and so on. Then I just took some screenshots of Unity scene with high quality settings (bloom, anti-aliasing, AO, DOF etc.) and slightly polished it in Photoshop. There I didn't have to think about performance - it just had to look good in a static view. Yeah - screenshot pimped up in Photoshop - rocket science level stuff ;).