Why can't you shift in the drops with Sora? it's designed for that.

For any upgrading on a lower end road bike, would be supprised if it isn't cheaper in the long run to sell it, and get a new one with the spec you need; 8 speed is obsolete on road bikes, Sora is now (MY2012 3400) 9 speed, and the part you want to change, the STI are normally the most expensive part on a road bike.

If you do change, and have never worked on a bike before, be prepared to take it to your LBS to get the bar tape wrapped, if you are going to do this a home, be prepared to have a few goes before you get it right.