actually, its like this;- tiagra=4xxx 105=5xxx, ultegra=6xxx dura-ace=7xxx , in the mtb world deore/stx=5xx, lx/slx/hone=6xx, xt=7xx, saint =8xx and xtr=9xx. they havent yet released an ultimate road bike groupset, they could do it any time, it took nearly 10 years to deliver xtr, so who knows?
Quality-wise, I think it makes more sense to think of Dura Ace and XTR as equivalent. They both used exotic materials in similar ways (Ti freehub bodies and cassette sprockets, for example) and both groups have tended to favour performance over cross-compatibility. The DA and XTR Easton seatposts were almost identical. DA and XTR were the first groups to use Hollowtech/Octalink, and the only groups to offer adjustable bearing versions.
XT and Ultegra are very similar in quality and finish, and make use of some design features from the higher groups (e.g. cassette spiders) without using fancy materials (steel freehub bodies, axles and sprockets).
I'm not up on the 10-speed stuff, but there was very little to choose between the 105 (5500) and Ultegra (6500) 9-speed groups. Component weights were very close. Finish quality was a little lower for the 105 group, some steel parts were black rather than bright plated, and one or two small parts that were metal in the Ultegra group used plastic for 105. The chainrings were cheaper pressings, and the cassette was spiderless. Apart from that, they were functionally very, very similar.
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De ce silence profond.