I am in the last phases of an upgrade to a nearly total 105 group. I have a fairly new XT rear der that I would like to keep, assuming it would allow me to use a cassette with larger cogs if I ever need it. My LBS buddy, a very serious cyclist in whose opinion on such matters I have considerable confidence assures me that 105 STI shifter will work fine with the XT rear der. I imagine that sheer physics would dictate that the shorter travel of the more compact 105 would make it at least theoretically faster than the longer XT mechanism. As a relative novice, I am not concerned with squeezing out the last 0.5% of efficiency, just reliable, quick shifting. As I say, I trust my bud, but sometimes I like a second opinion. I am perfectly willing to spend the money for the new 105 der. That $50 or $60 extra is not an issue at all. I just like the idea of the larger capacity. When my ability improves, the 27T cog capacity of the 105 may be plenty. But at this point I feel more comfortable know I can go up to 32 or so if I think I need it.
I don't NEED 105 stuff, but as long as I am spending the money, it's not that much more than Tiagra so I figure what the heck? Although I know theoretically how to do most of this myself, I am letting the LBS do it. I do most of my own maintenance and think nothing of swapping and even recombining cassettes. But some things can cause serious damage if done wrong so I figure paying the LBS is good insurance at this point.
And, yeah, I know I wouldn't be able to use something like a 52-32 gear combination, but I know better than to do that anyway. I only want the big cog for exteme situations. My regular cassette will be a 14-25 which is more than I need for my flatland commuting, but I hope to do some big hills/mountains sometime this summer and might need more. I don't need smaller than the 14 right now and I like the baby steps in gear inches I can get with 14-15-16-17-18-19. And, no, I don't care about the several duplications. And, yes, Travel Agent for the lever/brake mechanism difference. I think I have covered everything.