I run a hotter hydraulic cam and my doube (tandem) chamber power brake booster handles as low as the 5-6" vacum without any ill impact on the brake system.

Pedal remains stiff, brakes are activie all the time, grabbing very well, no problems.

This is not the 1st time I have done a hotter combo like that, my previous one was with a stock factory 11" power booster with a 7" vacum signal...no problems either...so they can certainly work in very low vacum situations.