There is nothing wrong with Tiagra. If the levers are 4600, any Shimano 8,9,10 speed derailleur will work, excepting 4700 components, and I believe specific DA level. I have used 4600 levers for years, matched with 6600 components, always solid performance. The big difference, is the 4600 has exposed housing. They work better than the 5700 I have, both smoothness of shifting and consistency. The first generation of Shimano STI levers, at least the 5700-6700, had issues with having to do a lot of tweaking for the shifting.