We've got a tandem with Tiagra STI shifters, Tiagra FD and XT RD. I was worried when we bought it because all of our bikes always have had Ultegra or Dura-Ace stuff but the Tiagra works perfectly, really no difference to Ultegra. I was pleasantly surprised. We did have some troubles with the FD dropping chains but a Third Eye chain watcher solved that problem. We also needed to adujst the FD and chainge it's mounting position slightly
As jnbrown mentions just be certain the FD is adjusted and set up perfectly. Our bike shop owner set up the FD with a very slight angle and it improved shifting after the adjustment.