matze -
It would be much easier to get into any given phd program if you have a solid contact at the school..
If you have the opportunity to make an impression on a professor, it's very possible to extend your stay for a PhD. I'd say it's especially true if you can show them that you are genuinely interested in their particular research projects. If UCSD is where you want to go, I would do it.
congrats, btw