Traditionally HDRi lighting is very inefficient with small bright light sources, but I believe the MLT algorithm handles them much better.
I expect Octanes' performance with them will improve considerably once Radiance finishes his RLT

Also to improve your space scenes, you should use a small bright light to produce nice sharp shadows. It may take longer to resolve but will look much more convincing.
| Desktop: GTX480 + 8800GTS 320MB, q9550, 4GB RAM | Laptop: Quadro FX 880M 1GB, i7 820m, 4GB RAM |