I'm about to do the same for my HTM. Been shopping wheels for over 2 months! Got a couple threads going on it too. I've always liked the old TC wheels, but they don't fit the XB. Now, these ones do and I'm stoked. Overall diameter goes up about half and inch or so. That shouldn't be too bad though. Offset is also less, so the stance looks a bit wider. Nice as well.
I'm also curious... Any changes in MPG or acceleration. My guess is you'll lose a bit of acceleration due to the extra weigh of the 18's. However, you also said something about K&N and also an exhaust.... So you might still be suffering in the MPG department, but made up in acceleration with those other upgrades. However, extra unsprung weight should still be a bit of a problem, but maybe not very noticeable unless you're over a very crappy road surface.