Diff gears in a stock car are a total waste of time. (personally think they're a waste of time full stop for a street car).
If you want to go faster, make more power.
With diff ratio's, the smaller the number the taller the ratio.
I.e, 3.08 is how many times the drive shaft will turn, to turn the crown wheel in the diff once. To make it clear. The drive shaft will spin 360 degree's 3.08 times to rotate the crown wheel in the diff once. Where as with a 4.11 gearset, the drive shaft will have to turn 4.11 times to turn the crown wheel once.
This means to do 100kmh you will have to turn the drive shaft faster, which means more engine RPM. Which means more fuel consumption, which means more expense in the long run. So really, over a tweleve month period, which modification is more expensive?
The cam or the dif gears?
(Given that it costs about $80-100 to fill a 75ltr tank at the moment, and you'll average a tank a week with a 3.08 gear set and probably double that with 4.11)