Agreed! Do it Rolfe! 
Not. A. Chance.
I'm actually deeply curious as to the reason for the price ticket on these things. After all, by taking the 18s, you're not having the 17s. Are the 18s really worth over £400
more than the 17s? What the bleep is actually going on here?
It's not the size of the 18s I find attractive. I appreciate that you geeks like the slightly bigger wheels but to me, I can't even see the difference. And if the extra inch radius makes the ride a little less smooth, then no way.
What I like is the shadow trim. If the shadow trim was available in 17s, for a reasonable upgrade price (it's only paint, dammit!), I'd probably have it. But paying £400 extra for a slightly cooler look (only noticeable when the car is stationary) with the downside of slightly degrading ride comfort, no thanks.
Rolfe.