I say just try the bullhorns with your current stem first.

As for the cables, you'll most likely have to get new ones. Not because they might be too short but because TT levers use the cylinder cable ends (usually used on MTB levers) where most road levers use the spherical cable end. Most cables will have both types of cable ends but to be safe just order MTB cables (or go to the LBS and pick some up there). Don't forget cable housing if you want to replace them too and ferrules.

I just put on the Nashbar TT bars and levers on my IRO build and i really like them.