In search for posts which document performance differences, very few, if any forum posters claim any improvements in speed with higher end bikes.
Personally speaking, I do not notice differences in shifting quality between tiagra 10, 105 11, and ultegra 11.
Even my tektro brakes seem to be breaking in nicely.
What you really want is a frame that fits you well, and to get a fitting that optimizes fit once you have the right frame size.
The performance differences between tiagra, 105, ultegra and dura ace are fairly miniscule. Add kool stop pads to any of those dual pivot brakes in any of those groups, and you'll have the best shifting and very good to excellent braking.
Paying more gets you less weight but you will not go any faster.