One difference between a road bike and tri bike is the angle of the seat tube. The angle for a tri bike is about 78 degrees and for a road bike is about 74 degrees (give or take a few degrees for each). This means that the steeper tube of the tri bike will make the top tube shorter so that your shoulders and elbows (but most importantly your shoulders) make a 90 degree angle. This will give the most powerful and aerodynamic position while keeping your chest open enough to breath well. Since the road bike has a shallower seat tube, the top tube will be longer, which is why you feel like it's such a long reach. In order to help this, they'll push your saddle forward on the rails. I think XLab makes a cocked seatpost that will make the effective angle of the seat tube steeper by placing the saddle even farther forward. This would be a better solution than buying a shorter bar stem since the steeper angle causes you to engage your quads and hip flexors instead of quads and hamstrings like the shallower angle does. That way, you'll have more power in the aero tuck and save your hamstrings for the run.

You should read this:

http://www.slowtwitch.com/mainheadings/techctr/bikefit.html