Replace Ultegra 6600 STI lever
The Ultegra 6600 STI lever is available as a right hand only part. It should cost around $400 dollars for the right shifter only, and around $600 dollars for the set.

Any Shimano 10 speed rear shifter, including the new Tiagra 4600 10 speed will be compatible. If you have current brakes, and you use a 10 speed STI lever, as previously noted, you will have no problem with the brake pull.

The compatibility chart can be found here.