The only problem with street use is that the extra usage tends to wear the pumps out quicker than race only applications. However I think there are a few out there that are more durable and made for street usage.
As for power... have heard claims of upto 30hp, but again that would scalable to the engines current power.
To be done right you need to build a motor for vacuume. They will make power when done right but not anything I would do on a street car unless it was a all out steet legal drag car.
I was going to say the same....build the car for it's inteneded purpose. 99% street doesn't need a pump. It will inevitably mess with your accessory setup.