Beside drive train, what is the difference between group components (e.g. 105 vs. Ultegra)? In his previous post, jmusits asked the difference between the Shimano 105 and Ultegra drive train. Obviously, the derailleur is certainly one of the major components of a group. However levers, brakes and cables also make a huge difference. I would like to extend jmusits' original question. 


What are the difference between 105 and Ultegra brake calipers? Is the brake performance better with the Ultegra components or is it only a weight question? What about the difference with Tiagra caliper?
What are the difference between 105 and Ultegra levers? I can imagine that the mechanism for gear change is more accurate and smoother for the Ultegra. But what about the influence on the brakes?


I am quite interested on the brakes right now, since my girlfriend owns a road bike with cheap Tiagra components. The derailleur is not working properly and we were thinking of changing the derailleur for a better one. But is it worth to change the brake calipers and/or the levers as well? She likes the frame, but would like to have better brake performance and smoother gear change. 

Thanks in advance.
Since you don't mention specific group sets (ie 6700, 6600, etc.) I'll say the primary differences are going to be weight and feel and to a lesser extent performance.

Ultegra 6700 levers are carbon fiber with a 'better' shape that some say offers better brake feel. Also the pivot point is different on 6700 vs 5700 (105) as it has trickled down from the Dura Ace design which theoretically gives more leverage and also requires less finger pull at the beginning of the stroke to move the brake pads when compared to 105 that don't have this feature yet.

As for the brakes themselves I believe the only noticeable difference is weight.