My customers and I would be pretty annoyed with only 6400km (4k miles) of chain life, thats why we use DA chains
I expect 10k+ km of chain life with a decent maintence/ lubing program.
They also consider them to shift better.
There are still 3, of the dozen or so that insist on full campag components and their arguments for the superioty of the Campag parts are the same as yours - Their Campagnolo, they must be better".
I'm not sure who "they" is (as they're not on this board!) but I'm speaking from my own personal experience (and other riders I personally know) and the published word from guys like Lennard Zinn, considered a bike maintenance expert and the paid technical consultant for Velonews.
Chain replacement is a subject that many rider's don't adhere to because they kind of don't have to.
The truth is that you can drive ANY chain longer then it's replacement life. Many riders do because they don't "fix it till it's broke", i.e., the drivetrain will start to make kind of a crackling noise and eventually start to skip. The "skip" point (as you say) is probably around 10k (6k miles). EVERYTHING will have to be replaced, chain, cassette and rings. I've done this (with a DA 9-speed system) so I know it's absolutely true.
Try to put a new chain on a drivetrain after it's recommended replacement limit and it will skip. Ahh, but it will work fine with the chain that it's been running with (probably true for your customers). It's because the chain has stretched and has worn the cogs. At that point the only chain that will work well with that cassette and rings is the one that has stretched because they have worn together.
If the chain is replaced within it's true limits you can get many more of thousands of miles from the cassette and rings then otherwise. So, either replace the chain within it's limits or replace the entire drivetrain ..
And.. to my original point (IMHO), the Record chain has longer limits then DA, about twice as long.
This is based on my own
personal experience and apparently there are experts that agree.