Yep, por si te funcionara, lo que tengo yo para activar el tap en mi touchpad (es un elantech, pero debería valer para synaptics también) en todo el sistema. Imagino que los bordes y eso los podrías comentar para que no modificaran los que detecte tu sistema:
Create el archivo /etc/X11/xorg.conf.d/50-synaptics.conf y mete dentro

# Example xorg.conf.d snippet that assigns the touchpad driver
# to all touchpads. See xorg.conf.d(5) for more information on
# InputClass.
# DO NOT EDIT THIS FILE, your distribution will likely overwrite
# it when updating. Copy (and rename) this file into
# /etc/X11/xorg.conf.d first.
# Additional options may be added in the form of
# Option "OptionName" "value"
#
Section "InputClass"
Identifier "touchpad catchall"
Driver "synaptics"
MatchIsTouchpad "on"
Option "HorizEdgeScroll" "1"
Option "VertEdgeScroll" "1"
Option "HorizTwoFingerScroll" "1"
Option "VertTwoFingerScroll" "1"
Option "TapButton1" "1"
Option "TapButton2" "2"
Option "TapButton3" "3"
Option "RightEdge" "3000"
Option "BottomEdge" "1900"
Option "PalmDetect" "1"
# Option "TouchpadOff" "1"
Option "SHMConfig" "on"
# This option is recommend on all Linux systems using evdev, but cannot be
# enabled by default. See the following link for details:
# http://who-t.blogspot.com/2010/11/how-to-ignore-configuration-errors.html
# MatchDevicePath "/dev/input/event*"
EndSection

Section "InputClass"
Identifier "touchpad ignore duplicates"
MatchIsTouchpad "on"
MatchOS "Linux"
MatchDevicePath "/dev/input/mouse*"
Option "Ignore" "on"
EndSection