My brother has a Trek 1200 with a Tiagra triple. He has several thousand miles on it with very few problems. The only real gripe is poor front shifting. I believe it's from having too short chainstays not the der or STI's fault. Trek designs their road bikes with decidedly race type geometry unless it's a touring model. The short stays produce a very severe angle in certain gears. If the bike were a double, I think the Tiagra would shift perfectly.
The only thing I have against Sora is 8 speed. Because the 9 speed standard was around for so many years, it is well supported. 9 speed is still the only option on a MTB. The choice of chains and cassettes for Shimano 9 is huge. Even though Shimano is phasing out 9 speed, the rest of the industry is still supporting it. 9 speed barcons are relativley cheap also.
Lack of support is why I am selling off all my Campy 8 speed parts. I don't think Shimano will ever be in that position. Good luck and you should consider one of the 9 speed groups if possible.
Tim