You can technically use filters on the 17, it's just very expensive; filter holder/slide filters/etc. And yes, the 24 is a better lens, but once you reach the high-end you'll be splitting hairs in quality. Also tilt-shift; never under estimate it's power, get a whole landscape in focus at f/8 from a couple feet to infinity..
In any case, now that I think of it, the "EF-S 15-55 f/2.8 IS" doesn't sound like a bad idea. It's not going to help push the OP over into full-frame land, but it does only cost about $1100, which is rather inexpensive compared to all the strictly-WA options mentioned and from what I've seen it's a very highly-rated lens. It also stops short of the 11-16, so there may even be reason to keep both.
It's really up to the OP now, go full-frame and have it easy with WA options, or get a second standard zoom that can go wide enough.