At least HD springs are required with all high performance camshafts and if the stock cam is to be used over 4,500 RPM (past the red marks on your speedometer in any gear) or if faster acceleration rates are achieved through more carburetion and/or a lightweight flywheel. I, personally, use single HD springs on all stock cams, and inner and outer springs with all of our performance cams on all sedans and bus engines. A starting installed height for street use is 1.500" to check for coil bind and total travel. A street spring needs about 0.150" extra travel from full lift to coil bind (make sure the retainer clears). Example: Valve lift at the valve 0.430" + 0.150" = 0.580" minimum available valve spring travel to coil bind. Because every engine combination is slightly different and has different requirements, this is one of the single most important requirements of making the engine live. We have the experience no one else has and are willing to provide the information at the time you are ordering our parts.
Please be clear that far more damage is caused by lack of proper spring tension as compared to too much as the valve is floated, allowing the spring to slam the valve closed. This will prematurely cause seat, guide, and retainer damage and is often the cause of broken valves. Furthermore, when the valve is floated it will not receive a full cooling cycle and can warp or burn far sooner. I, personally, have never found any damage related to too heavy of springs, ever.
Many people are not aware that valve float can occur at low RPM as well as at high RPM. This misconception has caused many engine builders costly failures. Float can occur at lower RPM when improper springs are used in relationship to the rate the engine accelerates. Many times I have seen engines in buses that had 5800 rev limiters with the heads beat to death from valve float. This is due to lack of proper springs, proper tension, and/or installation. After all, all of the late buses come with dual springs from the factory. I can assure you that VW would not spend the money to install dual springs unless they made the engine live longer.
Springs have an extremely bad harmonic problem with most standard ratio rocker arm (1.1) cams that are made for a VW. These require a dual spring that has the proper interference fit from the inner to the outer to stop the harmonics. In fact, many times it is not the need for the extra poundage as much as it is for the interference that counts. For more details about valve springs order GB 801-SPRINGS.