In the Gnome Shell and on Unity it seems that there is no tool or setting for increasing the mouse scroll sensity. But there is a old tool that maybe can help you called
Install it with the following command or from the Software Center:
$ sudo apt-get install imwheel
And edit the configfile
~/.imwheelrc with an editor of your choice (e.g.
gedit). Fill in the following for increasing the scroll speed for every Command:
None, Up, Up, 3
None, Down, Down, 3
3 is the increase of the scroll. For more specific scroll increases take a look to the manpage
$ man imwheel
or read the README of the project.
You can start it by typing:
be sure that you don't start the imwheel twice! That's a known bug, but you can stop imwheel with the command:
$ killall imwheel