Sportbike riders often add a larger back sprocket- the companies use a smaller one to more easily meet the noise and emissions standards, and these bikes go way too fast (higher mph) in first gear for the stock sprockets.
It is usually best to not add a smaller front sprocket- it wears the chain and front sprocket out faster, and may damage the transmission more easily.
If you do a lot of long, straight trips on a slower bike, a smaller back sprocket will drop the rpm in top gear and maybe make it run smoother, but it may either lower or raise gas mileage. A low powered engine may not have the power to lower the rpm too much.
If you do a lot of curvy country roads or drag racing, a bigger back sprocket will allow higher rpm in each gear, making the bike seem more powerful, but the top speed will be lower, and the comfort at 75 mph will be lower.
Adding a bigger front sprocket is like adding a smaller back sprocket, and most bikes do not have a lot of extra room in the front for a bigger front sprocket.
Aluminum back sprockets are for racers, to make their bike lighter, but they wear out faster.