i think it is must be, i don't know of any rule giving a limit to that. he's just stingy with + modifers as far as i've seen, and probably has a lot of invisible modifiers that aren't showing up, so that your +11 may be actually a +7 or so.
i got him to vote for me for diplomatic victory once and only once. it ended up being the deciding factor after a free religion change took away another voter from me. i was shocked, amazed, and quite proud ... toku got me a diplomatic win, without me ever going to war to make his smaller, but by actually earning his votes??? he wasn't at friendly but he was at pleased. i did have to get the visible number higher than usual for his vote because of invisible modifiers.
he'll sign DPs, but he'll never sign a permanent alliance even if you do get him to friendly. he just doesn't trust anybody that much. i sometimes get open borders when first meeting him, if i give him some cheap tech. i know he'll close them before too long, so i try to get a scout over there ASAP to see his land before the doors are shut until the trumpets of war open them.