If its sequential, can't one use a big turbine turbo for high RPMs and a small turbine turbo for low RPMs?
Also, I still don't understand why people say plugging a turbo into a car is gonna ruin the car. Its just recycling the combusted air that comes from the engine before exiting the car? If there were to be any damage it would probably be the back pressure from the turbo and it that case wouldn't the engine just stall?
What you've described is essentially a sequential turbo setup, which is what the RX7 came with originally...
'Back pressure' or turbo surge would cause compressor stall, which could result in possible damage to the turbocharger unit. Easily solved by using blow-off valve/ anti-surge valve.
When people say 'ruin' the car, I think what they mean is if the car is originally NA, turbocharging it would alter the 'character' of the car. I don't think they mean it in a literal sense.