Since nobody answered, the only way to find out was to test the "LTS Hardware Enablement Stack" and see if it works or breaks ...
I installed Raring's stack (kernal + Xorg) with the following command:
sudo apt-get install linux-generic-lts-raring xserver-xorg-lts-raring
And reboot ...
The system booted fine and almost everything worked, even some-things that didn't work before, such as brightness controls. But one major problem showed up: with every other boot, Xorg consumed a lot of CPU capacity (about %25), and the system was super slow and laggy! I don't know what was the cause, but it was either a bug in Xorg or a bug in the kernel module for Intel GMA-500 graphics.
So I thought to try Saucy's stack in hope of solving the problem, since it has a newer version of the Linux kernel. The thing is that the kernel-package of Saucy is available but not its X-stack. According to this link, it is not a good idea to use a kernel with a different X-stack but in my situation I had no other choice, so I gave it a try ... I installed Saucy's kernel with the following command, and Raring's X-stack remained:
sudo apt-get install linux-generic-lts-saucy
And reboot ...
Everything worked fine!
:) Now all the problems of my Xubuntu-12.04 installation on the Sony VAIO P with Intel GMA-500 graphics are solved without the need for any tweaking:
- The screen works fine with the standard resolution and adequate performance;
- External VGA port works and secondary monitors are handled fine;
- Brightness controls work;
- Suspend/Resume works;
Now I'm a happy Xubuntu user on my Sony VAIO P which is way more faster and responsive than the standard Windows-7 which came pre-installed on it!
Of course there is one limitation: the graphics performance on Linux is much lower than the performance on Windows. For example, I almost can't watch any kind of video format on Xubuntu, while all video files with resolutions below HD (720p) play fine in Windows ... Although this is a problem I can live with, it would be really great to sea this fixed in Ubuntu-14.04 LTS.
Finally, my general recommendation is that if practical, do a fresh installation of the latest version of Xubuntu (which currently is 13.10 Saucy). When the next LTS version is released (14.04 Trusty) perform an upgrade or again do a fresh installation. But if your situation is like mine and you can't afford a fresh installation right now, installing Suacy's kernel with Raring's X-stack can fix a lot of problems.