Once you get north of a certain amount, the diminished returns curve gets pretty steep. $3K is as good a deflection point as any. If you were racing and self-supported, you'd pretty soon realize that you could do a lot better with that extra $5000 than putting it into the bike itself:

race wheels
power meter
coaching
entrance fees
tegaderm

Not to mention that if you want to be competitive, you need to put in the saddle time, which means more money spent on consumables like tires, cassettes, chains, bottom brackets, handlebar tape.

On the other hand, a $8K bike isn't going to hold you back and unless you're missing mortgage payments it's not worth the angst of worrying about how people spend their discretionary income.