Hi Spectastic.
I know nothing about different group-sets and the sex-appeal of one versus the other. Just so long as you know that most of the branding and talk about bikes is mostly sex-appeal. I have a off-the-rack bike (my third road bike) which is a bit of a moungrel when it comes to its group-set: brakes are different to the shifters;both hubs are different; and the crank is Suntour which is also different to the rest of it. Bikes are only a collection of working bits afterall. But is does have Tiagra brake/shifters and Tiagra rear derailer and after years of training and racing on it I can find no fault with it. It also has Alex rims which I have found to be very strong after running over a large hole one day and thinking it must have put a flat-spot in the rim, which it did not. I hope this settles-down your worry that Tiagra is not durable or suitable for use with the racing stress and the many kilometres you will be doing. It may, however, not be to your liking in the 'street cred' area.
David