So, I've obviously never dropped $10k on a bike. But I ride a Look 695SR with full Campy Super Record and carbon wheels, which was $10k+ new.
It's noticeable.
That being said, working at a shop, I rode a lot of road bikes. I can tell the difference between a $500 mtn bike and an S-Works mtn bike, but I don't think I could tell the difference between two high end ones. Unless you're immersed in a particular environment, those super expensive bikes won't seem much nicer than normally expensive one.