I do the same thing: I buy Ultegra for the fast-wearing stuff, such as hubs, cassettes and chains, but Dura-Ace for everything else.
This is how I remember the 'history'........correct me if I'm wrong
they used the same chain width for a gazillion, years, right to 8 speed. Incidentally, when 7 and 8 speed came in they had to change the design of the hub body to the cassette style because too many axles were breaking -- I busted heaps of axles using 7 speed screw-on clusters. When 9 speed came by, they had to narrow the spacing (chain & cassette) because the chain-line would've otherwise been screwed up; same again for 10 speed.