My wife notices a big difference between the 105 and Tiagra front derailers. I can live with the Tiagra front derailer.
Personally, I always thought that the most important components were those with the ball bearings. Some not so scientific experiments--just rolling down a few hills with bikes that have very similar tires but different hubs--with Tiagra and 105 hubs led me to believe that the 105 hubs were considerably better. Bike Friday's Tiagra-level models often come with a Formula BF sealed bearing front hub. Considering the amount of work needed to change the hub later, I would upgrade that.
Also take notice of the type of valve used on the bike. I recall that the Tiagra model is spec'ed with a schraeder valve rim/tube instead of a presta valve. Not a big deal unless the rest of your bikes have presta valves.
The Bike Friday Yak group is big on the Chris King headset because of the stress of the long stem tube.
Are you thinking of STI shifters or barcons? If you go with barcons, I would place less emphasis on the derailers.
Considering cost, effort needed to modify later, and value added, my ranking of upgrades is the following:
(0) Chris King headset -- I added this in after typing #1-6
(1) capreo rear hub -- the gearing is so much better with the 9-26 cassette
(2) 105 front hub -- pretty cheap upgrade
(3) 105 crank/bottom bracket -- unfortunately, there is a compatibility issue with the Tiagra cranks and 105 bottom brackets
(4) STI -- one can often get good deals with a little patience
(5) 105 9-speed rear derailer -- are they still available?
(6) 105 front derailer
Note that a NWT with a complete 105 setup described above is about $2000. A Tiagra/Capreo setup is something like $1800. So you might have to choose your poison.
Don't forget that Bike Friday give a 5% discount to Adventure Cyclist and League of American Bicyclist members.
Good luck.