I think you are wildly underestimating the cost of parts, labor, and engineering that go into what you are describing.
Sure if people want to drop tons of money in the aftermarket they could end up with something faster. But for 99% of the population who will buy these the C8 is the better choice because
1) It has a readily proven value, so it's easy to finance and insure
2) It's easy to resell in the event they need the money, want to move to a different vehicle, etc.
Even if we assume price parity and say you can build a faster car for $70,000 that still requires that people have $70,000 that they can spend, be willing to tie that $70,000 up in something that they cannot readily liquidate, and are willing to accept difficulty obtaining appropriate insurance. For the vast majority of the population this is not remotely feasible. Financing a $70,000 car is trivial.
Based on the information we have so far C8 is an astounding performance value. It basically obsoletes any factory warrantied fun car under $250,000 unless you care more about the brand cachet and how hard the dash texture makes you vs what it can do.
Viper GTS