Here are the pros and cons of more gears.

Pro - More ratios allow you to keep the engine in the peak powerband through a wide range of speeds. You can also have a good low gear and top gear.

Con - More gears mean more shift points, and you can't accelerate while shifting. More gears in a limited amount of space also means a weaker tranny than less gears in the same box.

Here is also a case in point. My car is the last model that came with a 4 speed tranny. The five speed model that was identical except for the tranny had exactly the same performance numbers as the four speed. The 5 speed is also a weaker tranny than the four speed. My four speed is rated for 700 HP, where the five speed was rated for only about 450 HP.

Part of the reason why the 4 speed does as well as the 5 speed in the case of my car is since it has a wide torque curve.

Cars with peaky power points are more likely to benefit from more gears.