The reason you need a veriner pulley is to get the valve timing exacty correct. As suggested this can be affected by head skimming.
More importantly the cam of choice may max lift may be at 106 deg atdc (Piper PB285), where as stock is at 112 deg so a vernier is required to retard the timing to the correct place.
The Newman cams are designed to run on stock timing marks removing the real need for a veriner pulley @ £80/90.
The longer the duration the more focused the power delivery, 268 is a good road cam in a fairly standard engine. 272 is ok works better in a ported head and free flowing manifold. 276 is realy for race or rally in a big valve head with manifold etc etc Very focused power delivery. When looking a forced induction camshafts lift is needed but not too much duration, go for a 264 deg camshaft.