Look, any fool knows (or should know, anyway) that the gains that can be made through buying high-end equipment are much smaller than the gains that can be made through improving one's fitness. But the cutting-edge tech and gear is a part of the sport that interests and excites a lot of people, all out of proportion to the difference it makes in how fast they're going. Having nice stuff is fun! And nice! And I just think you're totally out in left field thinking that whatever speed people are able to buy will be "unsatisfying," as though you can't both buy faster wheels and improve your fitness. These aren't mutually exclusive!

Common sense? You say common sense, but what I'm reading looks more like this:


Like I said: having nice stuff is fun! It may not necessarily be justifiable - hell, I race, I know the value of equipment and I sure as heck can't afford carbon wheels or a Di2 group. So I concentrate on the stuff that's both affordable and that will make the biggest difference for me personally, things like contact points, shoes, and so on. But I'm also using Ultegra and not 105, because I like it better. And I have some wheels with nice Chris King hubs, because they're pretty light, and awesome. They might not be much faster than Shimano hubs would be, but they're sick! "Common sense" by your definition sounds awfully tedious.

Of course, stupid threads like this one don't really help, and few people are able to give the right answer, which is that the question is unanswerable. It just depends on too many factors, for example, like what it is that even makes a bike nicer. I assume that generally means "more expensive, with higher-end components and lighter weight." In my case, my bike is nicer than the last one because it fits me better. And I do go faster on it, though that's mostly because I'm fitter. So a nicer bike probably will make you faster, but "how much" is unanswerable. Probably not by much. But that doesn't mean you shouldn't bother buying one.