Looking at your lens inventory, the first thing that comes to my mind is if you move up to FF right now, then three of your lenses will be useless (on a second reading of your post it appears that you know that). Definitely, 5D markIII is a fantastic camera and going from 550D to 5DIII will be a great update. But, my photography philosophy says that the quality of our photograph depends to a large extent on the quality of the glass that we use. If I was in your position I would sell of 18-55 and 18-200 EF-S. First, they are EF-S and second they are slow and third as much as I have read they are optically quite poor performer. 10-22 apparently is a better glass and since it suits 550D, I would keep it. So now you have a 10-22 to cover the Wide Angle side. A 35mm (I think your 30mm 1.4L is a typo) to cover the normal angle of view and a 50mm for the short telephoto angle of view. Then I would look at the photographs that I have taken and see which focal length have I been using most for my landscapes most? If I have been using my 18-200 more for my landscapes and that also the longer side of it, then I would buy 70-200mm 2.8L IS II. I could have gone for f4 but then for night photography it would be pretty limited. On the other hand, if I have been using 18-55 more for my landscapes then I am in a bit of dilemma. The best wide-normal zoom for 550D would be 17-55 2.8 IS, but that is EF-S. Then I will use the logic that photography is my hobby and I do not earn much from it (may be nothing at all) and so will go with the 17-55 2.8IS. Therefore, although I will not have the "grand" 5DIII, but I will have five of the fantastic quality lenses that canon has to offer -
17-55 EF-S 2.8
70-200 2.8L IS II
Then towards the beginning of next year (or end of this year) I will check again and see (reevaluate) where my photography stands. If I have money I would upgrade to a better cropped sensor (still all my lenses remain good for it) - may be 70D, may be 7DII if it comes out. Gradually I would gather funds/save for the 5DmkIII and the 16-35EF (which you will have to buy for wide angle on FF) - it will come, but not just right now.