Front derailleurs are very simple and I doubt if there would be a noticeable difference from Tiagra to Dura-ace, except maybe weight and the idea that you've got "the best". However, the R800 should come with full 105 and you should either get it changed over to 105 or maybe an additional discount if you don't mind the difference.

Are you going for the double or triple? I originally ordered my R800 in a double, but there was some confusion and it was built as a triple. I rode it for several days before taking it in for the conversion to double, and I found the shifting performance increased greatly with the granny ring out of there.

Unless you live in a *very* hilly area I would suggest the double. I'm thinking about changing over to a larger cassette, so that I will still run two rings up front but have lower ratios. Even on downhill sprints I rarely use some of the really large ratios, but there are times where it would be nice to have a little lower gearing. ;-)

The R800 is an amazing bike, congratulations on an excellent choice!

Good luck!
Mike