It's common to have a mixed group to lower the price and everyone does it. BD has a model that is one price point above the mixed Ultegra and that model has full Ultegra for a mere $200 more, plus slightly higher grade Ritchey Logic components. That is the best upgrade ever because you can't replace those components with the $1800 bike for a mere $200. If having a mixed component bugs you, go for the full Ultegra by spending that extra $200, which is peanuts for a Ti bike.