RSX and RX100 stuff is very good quality for lower-end. I used RSX brifters on my Pina for a few years and I still use some RSX brake calipers in a couple of my bikes. Good polished finish, sturdy construction. Not sure how they compare with Sora and Tiagra, but definitely useful. Yeah, 105 or Ultegra would be a nice upgrade. Start with the rear-derailleur. You can get a 105 rear der. for $25 on e-bay.