1. for Rear wheel drive cars with lost of horsepower, you might need to lower down the final drive to reduce the burnout. but for 4 wheel drive cars you should increase the final drive to get a bit of a slip on the wheels so that you could get max grip with max power. that is key, gear ratios are really good the way they are until you are like trying to get an extra .0001 seconds faster.
2. IF you adjust your gear ratios, make sure your car accelerates smoothly and constantly not just at high rpm's. this means that your car will keeps its speed until hitting 7k rpm and then accelerate, and then you shift and it sits again. If you have this problem increase the gap between the gear rations that you are having this problem with.