Kinetic two-finger scrolling is called "conventional coasting" in the Synaptics driver:
Coasting comes in two flavors: conventional (finger off) coasting, and corner (finger on) coasting. Conventional coasting is enabled when coasting is enabled, and CornerCoasting is set to false. [Synaptics manpage]
The two modes are mutually exclusive: kinetic two-finger scrolling does not work with corner coasting enabled. So let's enable conventional coasting. As per the new Ubuntu recommendations in the file, do not edit your
50-synaptics.conf directly but create a directory
/etc/X11/xorg.conf.d/ and in it a file like
Option "CornerCoasting" "0"
Option "CoastingSpeed" "20"
Option "CoastingFriction" "50"
To see the effect, restart X.
As you noticed, the speed and friction values are hardware dependent. What works on one touchpad might not have a noticable effect on another. Some experimentation might be needed.
For experiments, it is better to use
synclient, as it creates an immediate (but non-permanent) effect without having to restart X all the time. The equivalent to the above would be:
synclient CornerCoasting=0 CoastingSpeed=20 CoastingFriction=50