It mostly comes down to achieving your best fit but also the handling of a dedicated Tri/TT bike can be better than a converted road bike depending on your position. A TT bike is designed to be ridden in a steep position with a lot of weight on the front end. If you ride a similarly steep position on most road frames with a fast forward style seat post you're really changing the weight distribution relative to how the bike was designed to be ridden. That can end being a really squirrely ride.
Dedicated TT bikes also tend to be a bit more aero but your S2 is no slouch in that department.
But if you can get the stack and reach you need on your current bike and like the way it handles in your best Tri position then you can certainly go that route. Realistically swapping back and forth from an aero bar to traditional road position is a PITA and not something folks want to mess with on a daily basis.
So one advantage of having dedicated road and TT bikes is you jump on one for daily training and your position is dialed for that type of riding and next time you get on it you don't have to spend time making sure it's set up correctly. This usually means folks actually train more in the TT position if they can do so on any day without a lot of setup time. That assumes you ride the road position frequently of course the converse is true, if you mostly ride in the tri position and your buddies want you to join a hilly group ride or weeknight training crit you may or may not want to (or be allowed to) roll the tri bike and then have to decide if the switch over is worth the hassle.
Just signed up for my first ironman. Done a few sprints, oly, and half with my cervelo s2 with clip on aero bars and 2 way seat post. I was thinking about doing exactly what this op did to his S2 http://forum.slowtwitch.com/...ew_flat;post=3980453
Or should i get a used p2/p3 instead? I would like to know what are the cons of doing a conversion besides having to switch the cockpit between group/solo rides. Are the angles slightly different where i won't benefit as much versus getting a full on tri bike? Still new to this sport.