[dsraven]

personally, I would use a hydroboost under floor rather than a dual stage vacuum style booster. it will give you excellent boost the same as it would if mounted on the firewall. a dual vac booster is dual because a single doesn't give enough boost due to square inches of diaphragm available. yes, lots of vehicles are using the dual vac booster. I would say try driving one and then try driving a similar vehicle with a hydroboost. you will see the difference. a hi perf engine can also make a vac booster less efficient as there is less vac at certain rpm's compared to a stock engine. do as much homework as you can before pulling the trigger. more complicated means longer time till driving.
again, just my opinion-not worth much really