I wouldn't call your 18-55 pitiful -- it is sharper than the 17-40, despite costing 4 times less, including I.S., and more range. Using the 17-40 on a crop camera will be a downgrade from your existing 18-55 in several ways. It really shines on full frame though. As for the choice between 17-40 and 16-35, it mostly depends on what f-number you need, IMHO. If you'll be doing f/8 on full frame, it doesn't make much sense to shell out for the 16-35. But f/2.8 is very well worth it for low light IMHO.