Originally Posted by Kimmo

I'd suggest not. As long as the old gear isn't shagged.

But I guess it depends on your criteria. I'm pretty sure there haven't been any advances in derailleur geometry since the slant parallelogram (the newer one might have bigger jockey wheels, which is a slight improvement), and the only change I'm aware of to STI aside from adding speeds is the redesign of the hidden-cable models, which Tiagra isn't one of.

It may prove in practice that the new shifters and the old derailleur are better, or vice versa - IMO you can't be sure until you try em.

I'd be amazed if the 105 gear wasn't at least lighter and better-looking.

Also, when it comes to crisp shifting, I've noticed many derailleurs have play, even from new, in the upper sprung pivot. I've fixed it a few times by putting a washer behind the circlip (on Shimano derailleurs I had to file a notch in side of the washer, on the Campy one I had to grind it thinner) with good results. Finding a way to add tension to the parallelogram spring is often well worth it too. Also, on Shimano derailleurs I piff the Centeron pulley (or you can just swap the pulleys, that should be 95% as good) - if you think about it, that lateral slop in the upper pulley is kinda like a dork disk - it detracts from a properly-adjusted machine.