I had asked my friend the same question in May. We were at an MS ride and in the registration area there was a local bike shop with a few top-end bikes. He started talking to the guy, and they both agreed that the only real difference between 105 and Ultegra or Dura Ace is the weight. In their eyes, 105 was a good enough system and unless you were racing and would actually benefit from the minimal weight difference, there really was no point in upgrading. I felt a little better knowing I had a decent setup and that any perceived issues were probably a direct result of me just not being a good enough rider. Why are you concerned about the brakes specifically?