I would not intentionally plant a bigleaf hydrangea in full sun in any zone. Regardless of what the plant tags may say, these are essentially woodland plants and need a part or dappled shade environment. Morning sun & afternoon shade is fine.

Even in my very mild and far northerly zone 8 location - where the sun is not very intense and it never gets too hot - bigleaf hydrangeas in full sun wilt on a daily basis.