Sora is a basic, low cost level of parts that tries to include as much of Shimano's designs as possible using material changes and tolerances increases to save cost. This is not good or bad, just what it is. With care and feeding it can last a long time, without, parts will wear down and loosen up at a faster rate then more costly part levels. I am surprised that Sora would be sold as a "group" though. Parts packaged and marketed as a Group are typically found in the higher levels and the basic stuff marketed as individual replacements only.

I will say that from my experience Sora is a vast step up from 2300. Andy.