How much lighter a purpose-built Lexington might have been

What displacement might Lexington have had if designed as a carrier from the beginning?

A rough estimate is 27,000 to 30,000 tons standard, instead of about 37,000 tons for the conversion.

Lexington displaced roughly 37,000 tons standard as a converted battlecruiser. A qualified estimate puts a purpose-built carrier with broadly similar protection at about 27,000 to 30,000 tons. The saving would come from omitting structures and weight inherited from the battlecruiser design rather than from stripping away every survivability feature. This is a reasoned range, not a detailed reconstruction from complete plans.