So how useful were large destroyers?

I have long been enamoured of large, fast destroyers. In 1905, 1200 tons would be considered "large", although the British Swift was much larger than that. My friend Cliff's flotilla leaders (DL's) were 12oo tons. His larger fast gunboats (PGL) (destroyers without torpedoes) were 1600 tons. The plan was to use these in place of light cruisers. The PGL type would have had 4-5in/50 QF guns, able to fire a four-gun broadside. That would have been sufficient to outgun many light cruisers prior to WWI.