Saying Potential GDP to be not preferable cuz it’s not actually efficient isn’t that different from “nominal output of machines is useless (or not preferable) because it costs more to eliminate losses so that you can reach there”. That’s how your words sound like. What’s the point of arguing ‘max potential’ to be inefficient except for nitpicking?
Sales Tax or Value-Added Tax reducing the GDP doesn’t mean the consumption is factored in its calculation. If you regard that as a proof, you can also say pollution is a source of GDP since Pollution Control also reduces the GDP. The fact Sales Tax has a negative impact on GDP just reflects losses incurred by taxation in the markets. C+I+G=Y just means parts of Gross Domestic Product are consumed by the people, not consumption contributes to the production (GDP) directly. You need to produce or earn more goods & services if you want to consume more anyway.
I’d also like to remind a comment written a month ago.