It is the cam shaft that governs the amount of lift that a valve has. The greater the valve lift, then the greater the lift rate will be. With a performance cam (or height lift cam) this lift is increased allowing more FAM into the combustion chamber. This results in greater torque and power at higher revs for more aggressive cams. When the engine is pulling low revs the modified cam would result in a slight decrease in torque. The benefits to be gained by increasing the lift rate and the valve lift are generally greater on a larger engine rather than a smaller one.
When changing the cam and its lift characteristics, up-rated valve springs should be fitted, thereby matching the valve springs to the up-rated cam. If the original springs are used then the greater cam lift could cause the springs to become coil bound. With a more aggressive cam and high revving of the engine there comes a point where the inertia of the valve and its components are too great for the valve spring to counter act. This in effect changes the timing of the engine, and is known as valve float. Up-rated springs counteract this greater inertia. However if the valve springs are too stiff this will generate increased frictional losses and as a result more BHP will be lost.