Roof rack redux - Yakima Whispbar

I've seen several threads in these forums regarding roof racks. It seems like two of the main complaints are the hit on MPG and extra noise. None of the threads, though, talk about the Yakima Whispbar which supposedly reduces drag and noise considerably. Does anyone have any first-hand experience with this system? I'm considering the roof rack option (due to frequency of use) for my soon-to-be 2006 Prius but don't want to drag down my mpg too much.

Also, I'm wondering if there is anyone who has tried to have 4 bikes and 4 passengers in their Gen II prius. Given that I can get all of the bikes on (two on a roof rack, two on a temporary trunk rack), should I be worried going over the car's capacity to carry stuff around? Forgive me for being a bit naive perhaps, but I'm used to my former SUV's capacity and still not familiar with what the Prius can do.