It all depends on the environment you use the parts in. A lot of race and track cars are only outside for relatively few days or weeks of the year so never see significant UV exposure. In most of northern Europe, the mixed weather again means the UV exposure is not that bad - we have seen bonnets on cars for almost 8 years now that have been kept outside and show no sign of yellowing.
If you are in the sunny states of the USA or the middle east, the massive increase in UV exposure means that you may well be better off having the part lacqered with a UV stable clear coat.