I always thought my 15-85 was wide enough, but i've been gradually coming to the opinion that it's not. ANd I haven't even been going anywhere special, just hiking around the local hills and the last week in Sydney.
But there's also stitching, of which I do quite a lot. Firstly, I stitch a lot with my Samyang 35, it's practically distortion-free (on a crop body), and it's sharp as all hell.
But I have also taken shots to stitch with the 15-85, and it depends what program you use (and how much you overlap the shots), but the barrel-distortion can be a bit much at 15mm. Zoom it in a tiny bit to 18-20mm and that's where the lens goes from barrel to pincushion, there's a practically distortion-free point somewhere. But you'll need a good tripod and head, a pano-setup becomes more and more necessary once you get wider and wider because you'll have more things closer to the frame where parallax becomes a problem.
Or just get the 10-22, take care of it, sell it once you're back (if you don't love it enough to keep it), and you're down less than the cost of hiring it...