Well it all depends on what you want to do and how "serious" you want to get in terms of TT'ing. Typically with an aero setup like you are talking about you will be more stretched out, so bringing your body closer to the bars may be a bit more comfortable. But a shorter stem is going to make an already twitchy position (being on aero bars takes some practice) a little more fun. Maybe a forward seat-port would be a better option, plus there is some good data coming out now that a steep seat angle = a faster TT (http://www.slowtwitch.com/mainheadin...seatangle.html)

And why keep the STI shifting? You can probably unload the brifters and cover the costs of some 188's and bar ends and most of the bullhorn bars. As to aero bars, the Syntace C2's garner a lot of love, especially in a bullhorn setup.

Next step would be some wheels, you can pick up a Hed H3 for around $300 if you look hard enough, then pick up a CH Aero wheel cover (around $60 or so) and you'll have arguably the best aero wheel setup around (may be a little heavy, but it will be very aero).

Or you can pick up a set of Profile Jammer GT's, slap them on and go enter a TT

Andrew