the diff gears act as a torque multiplier.
in a diff with 3.08 gears, it takes 3.08 turns of the tailshaft to make 1 revolution of the wheels. so when you change the diff gears to 3.9's, it takes 3.9 turns of the tailshaft for 1 wheel revolution. that means that the torque will be multiplied more than with the taller (lower number) gears. when a car is geared shorter (bigger number), it will accelerate faster but wont reach the same top speed. just like when you put a taller diff ratio in, the car will lose some acceleration but gain more top speed.
the reason people change their diff gears is usually to gain more acceleration. in commodores the standard ratio is around 3.08 (depending on the model of course). so they will go to 3.45 - 3.5 - 3.55 - 3.9 - 4.11 to gain some more acceleration without any engine mods. the car will then rev higher at the same speed on the road than before (example my vs ute revs at 2055rpm with 3.08 gears at 100km/h, but if i changed to 3.55 gears it would do 2369rpm at 100km/h). this will possibly hurt fuel economy a little but but the car will accelerate faster than previously and 'feel' more responsive on the throttle.
commodores are geared a little tall from the factory to get a good fuel economy, rather than performance. they are usually geared that they will never reach the rev limiter in top gear so its a bit of a error if you are going for the best performance. using my ute as an example again, it will be able to do a 'theoretical' top speed of around 270km/h in top gear on the rev limiter. we all know that there is no way a standard v6 will ever get it moving that fast, so when you change the diff ratio to a shorter one the maximum top speed drops, but it will be more achievable. if it had 3.55 diff gears instead of the standard gears, it would have a theoretical top speed of around 235km/h, which would be more realistic.
so in reality, going for that change in ratios wouldnt affect the top speed of the car as it never could reach the maximum speed of the gearing anyway, but it increases the acceleration. but the downside is higher rpm when driving at a given speed so fuel economy may worsen.
hope this helps.