I have seen some pictures of people replacing their OEM valve covers with something aftermarket, usually something painted. Considering that is only pretty much just a lid that goes over the topmost part of the engine, camshafts, and has all the necessary sockets for sensors, FI etc, I was wondering if such an upgrade can actually yield an engine performance enhancement or is it just done to make the engine look more sporty.
As the name implies, a valve cover is just a cover; its function is to protect the innards from the outside elements. It has no direct impact on engine performance unless the original valve cover was damaged or cracked to begin with.
That said, replacing a worn valve cover gasket on certain engines is probably a more effective way to recover lost performance as it can prevent unmetered air from affecting fuel trims (not to mention avoid unwanted oil leaks).