First, to conserve memory, X caps itself at the maximum resolution of the monitors it sees when it's starting. You fix that by adding the "Virtual" section to your Display block.

And in the "Device" block I added a few performance tweaks I found on the web.

It's still not as snappy as I'd like (and as I'm sure it can be). Anyone spot anything else I can add?

Section "Device"
        Identifier      "Configured Video Device"
        Option          "AccelMethod"   "exa"
        #Option         "AccelMethod"   "xaa"
        #Option         "MigrationHeuristic"    "greedy" # mess up systray
        Option          "FramebufferCompression"
EndSection

Section "Monitor"
        Identifier      "Configured Monitor"
EndSection

Section "Screen"
        Identifier      "Default Screen"
        Monitor         "Configured Monitor"
        Device          "Configured Video Device"
        SubSection      "Display"
            Virtual     2464 1500
        EndSubSection
EndSection

I'm amazed at how small the config is these days. Nearly everything is autodetected. I'm running the external monitor at 1440x900 and the laptop at 1024x600. Once you get the Virtual setting in place, the packaged GUI works fine.

If you are having trouble, I suggest you try out xrandr. It provides the error messages the GUI tends to suppress.