Hence the second part of my question. How much 'better' is this mythical top end machine, than something more economical?
It's a very fair question and the answer is probably miniscule to non-existent, or even a negative from certain angles, depending on whether you're looking at performance or longevity of parts. But, not to be underestimated, there is also the mental/placebo effect. If you skimp, or think you've skimped, on any parts you may always be left wondering if it's you or the machine letting your performance down.
If you're racing or take your times/speed very seriously then you can make a justifiable case for top end gear
If you're not then the only justification is because you want it - a view I see nothing wrong with - for example I drive very little now and rather than spending my usual £20k on a replacement car I bought an old runaround for under £2k(if I was braver I wouldn't have bothered at all) - that frees up a lot of bike spend, though a fair bit of mine seemed to get diverted into patios, patio furniture, etc.
But I've taken the longevity and versatility options over the bling and performance, I hope I've still retained a little bling though. For me that means Rohloff, SONDelux, Ti frame, disc brakes, strong handbuilt wheels, etc. I don't expect any performance gains from what I've done but I do expect savings around longevity of parts and batteries for lights.