There are another couple of factors than just 'performance' - reliability & longevity.
The lighter/more rigid etc. materials as you move up the range combined with (possibly?) better manufacturing and finer tolerances could suggest improvements in these at the top-end.
Our LBS, however, has a low opinion of Dura-Ace for The Common Man (as opposed to The Racing Man) because it is so finely crafted that it's not so hot on taking abuse from sub-optimal maintenance/being dropped.
You are indeed probably best off starting from the shifters and working down the drivetrain; having experienced new Ultegra vs Tiagra, the former is much better than the latter (though the latter is actually pretty darn good).
Matt