I love my Tokina 12-24. It feels solid and it is cheaper than the Canon EF-S 10-22 and superior in some aspects. Correct, the 12-24 and many other ultra wides are only for crop sensors only. If I was in your shoes, I would use the 17-40L.nice shots of a nice beamsicle dude. i just looked at what you have for lenses. i hear nothing but good things about the tokina 12-24. i'm looking at getting a 5d to compliment the 40D so i dont know if the tokina would be a good choice seeing as its a digital only. i assume you dont use the 100-400L much haha. pos some pictures done by the dust pump man!
The 100-400L is on a long term loan and I have used a couple times for birds and for some surfing shots. It is not very useful for cars at all unless its at the track. From what I have read, the lens is highly versatile as it has no equals. There are primes like the 400 and 600 but those are alot more difficult to use due to the fact they are primes and they are alot more expensive.