Stock springs have 78-82lbs of pressure seat pressure on average at 1.380". If you are over 50psi (boost or drive presure) and 3,800 rpm, stock springs do not fully control the valvetrain. This allows a few negative things to happen. First, valve float, which can cause serious damage to the valvetrain. Second, intake and exhaust valves can be blown open by boost and exhaust drive pressure which allows reversion. Reversion can hurt your power potential by allowing exhaust gasses to enter the cylinder on the intake stroke. Inert gasses in the exhaust, when re-introduced into the cylinder shield the fuel particles from oxygen in the cylinder.