It depends. Do you think you may have a need for the stock groupset down the road? Maybe for a commuter, a rain bike or one for the wife/GF? If not, sell the bike as a whole and take that $400 you were gonna spend on the 105 group, add the $700 from your current bike and look on ebay. For $1000-$1200, you should be able to find a pretty good used carbon bike with 105 or Ultegra on it.