Don't get me wrong - proof is important, but it's not "the greatest treasure-trove of leaked information ever, in history".
If you want an actual leak, then perhaps Watergate? Pentagon papers? The Treaty of Guadalupe Hidalgo? Even Wikileaks own Afghanistan War Logs.
In the face of all those the cable leak fades a bit.
Don't get me wrong - proof is important, but it's not "the greatest treasure-trove of leaked information ever, in history".