[Christ]

A tunable (on the fly) exhaust could alter performance curves, though.

Backpressure in an exhaust is never desirable unless it's a design function required by the engine's characteristics. The average engine doesn't need it, but some designs prefer it at certain ranges, and there have been obscure designs (I don't have a link) where exhaust throttling was used in place of intake throttling to control engine speed and power.

I have no idea what the significance of doing so would be, and don't ever intend to test it. I'd prefer to go throttle less altogether.