How reports of Gloire shaped Warrior and Black Prince

Did a navy ever start building against an enemy ship that intelligence later showed was less capable than expected?

Yes; reports about Gloire helped drive Warrior and Black Prince, which were completed even after better intelligence showed they already had ample superiority.

Early reports of Gloire's intended capabilities prompted plans for a Warrior design that could be expanded into a two-deck ship. That option allowed Britain to increase firepower if the French ironclad proved as powerful as advertised. A detailed intelligence briefing reduced the apparent need for that escalation. Warrior and Black Prince were nevertheless completed and substantially exceeded Gloire's capabilities. The case shows how better intelligence could change the required design without ending the construction programme.