A vacuum pump creates a vacuum in the crankcase to enhance ring seal, it also helps by allowing the piston to move down without pumping as much air in the crankcase. That in combination with low tension rings will free up HP, more than an evac system can ever do. There are gains even with standard rings. 10-12 inches is typical, more and you risk drying the components that rely on splash oil such as piston pins. Last, sealing the engine is a key to making gains. Dipstick tubes, leaking gaskets etc will allow air into the engine and diminish any gains.