But if you have the money to spare, why not?
A $10K bike will absolutely fly, even with me riding it. And it will look great. And it will start conversations and draw admiring glances. Well so you hope, I guess. But the law of diminishing returns applies to bike hardware, big time. That last extra $1K you spend gets you maybe 0.5% more performance. And the preceding additional $5K got you 2%. If that.
Well 2% extra oomph is still worth having, isn't it? You'll win with that, surely? Well, it depends. A $4K bike - or even a $2K one - will be almost as well built and reliable as a $10K one. Sure there will be differences, a top-name brand may indeed be better put together and will most likely weigh less and slip through the air easier, but the differences aren't as major as the marketers and magazine reviewers would like you to think.
A human bieing pedalling a chain driven bike is very efficient anyway and there is little to be gained from the transmission alone. And a triangular frame of any material is stiff by design, so any stiffness (and concomitant power transmission) gains are tiny. And if you think that crank stiffness really matters then you are doing too much time in the gym or are a track sprinter, or both.
You may get some aerodynamic gains - but these don't really matter below 30km/h and only pay off significantly over 40km/h. But even these gains don't really matter unless you are in a solo break, time-trialling or fronting the bunch over long distances. If you are drafting then the savings are non-existent to vanishingly small. But if you are a solo-break kinda rider, especially one with a big personal frontal area, then it may matter; if only to give you more confidence and motivation to stay out there and fry.
You will get some comfort gains, perhaps, or improvements in feel, and maybe - just maybe - better handling from a more expensive bike. But all bike designers know how to make a bike handle, it's not really a black art. And handling will vary with your personal setup and skill in shifting your weight around on the move. So a cheaper bike will likely as not perform much the same as a top-spec one. You may notice a difference when you swap from bike to bike but whether it helps you in a race is debatable. It may all be in the mind.
Mind you, I can't talk: I spent $5K on a Look carbon bike in 1990. That was a lot back then and I thought it would revitalise my racing and training. It was a motivational aid, if you will, and it worked for a while. Until it just became another bike. And then I swapped the groupset to my favourite steel-framed ctit bike and hung the carbon frame up for a while. Mind you, unlike most of my other bikes - and especially the steel ones (all rusted out or sold on) - it's still in my hands and rideable. And it remains light and fast. So it wasn't a bad investment, really.
But did I need to spend top dollar? No. My winningest bike was my first "race" bike, or first bike I raced anyway. It was steel and made in Japan. It was a Shogun. It had mid-spec Shimano components. It was around $700 new in the early '80s. It worked. It won. And as I went up the grades I ugraded from clinchers to tubulars (glue-ons or singles). It went faster still - and still won. But it really wasn't the bike, it was me.
I was motivated, racing was new and fresh to me and I liked winning. So I trained hard and raced even harder. Although I "upgraded" the bike many times (to Gitane, Colnago, Look and a semi-custom steel frame) and went as far as I could go in the local crit grades I never really did any better than I did on that relatively heavy, low-end Shogun.
And to be honest the biggest improvement I ever made to a bike was swapping from clinchers to tubulars. If you really want to go faster, invest in better wheels.
Now if spending big on a bike motivates you to train and race then so be it. Spend the money. You may not get the performance enhancement you expected but you may get a nicer bike that will last longer. And - most importantly - keep you cycling.