The reason you buy a cam gear is because it's built to be easily adjustable and maybe even slightly lighter than stock. The stock one might be adjustable (I haven't looked at it yet). Even if it is, then it's not as accurate as the one from AEM.
Some people say that cam gears for our cars don't do anything. I think they're not explaining that properly. Remember that the cam gear is tied in by a grooved belt to the crank shaft. As the crank shaft turns, the cam shaft does so as well. The cam shaft controls when the valves open and close. The valves let in fresh air and let out exhaust gases. I think the amount of fuel that gets injected into the engine is also tied in to this process (not sure though).
In a SOHC engine, you can make the intake and exhaust valves open sooner and close sooner, or open later and close later by adjusting the cam gear. This basically changes the power curve of the engine, but does not increase the horsepower. In a DOHC engine you have a greater control over these valves because one cam gear controls the intake valves and the other cam gear controls the exhaust valves. Then you can have the exhaust valves open sooner and the intake valves close later. That way you can adjust the power curve and actually increase horswpower too. | 