When It comes to legato speed , I would recommend you no less than the master of legato, which is ALLAN "the Man" HOLDSWORTH.

you can buy his instructional video or download some of his songs to hear what I mean...check out this songs all done in legato....you will notice the hornlike guitar sound with the silky smooth legato runs played at blistering SPEED!

Red Alert
Devil Takes the Hindmost
Road Games