A 15,000-ton cruiser near the battlecruiser boundary

Would a 15,000-ton cruiser with twelve 9.2-inch guns count as a battlecruiser?

Not quite; it would still be a cruiser, though one close to the battlecruiser boundary.

A proposed treaty-era ship of 15,000 tons with twelve 9.2-inch guns would still be a cruiser, though close to the boundary leading toward battlecruisers. Large displacement moves the design along the spectrum. Heavy guns push it in the same direction without automatically changing its type. Role and overall design remain more important than a single tonnage or caliber threshold.