Congrats on yr new ride!
Know one chap did exactly that. http://forums.e60.net/index.php?showtopic=334&st=2370&#entry110771
Going from 16" to 18" will give you harsher ride. I have dumped my end of life runflats for Pirelli P-Zero. Less harsh than run flats and of course less crispy steering feel now.
Drove a loaner 520i from PML for a week, the 16" tyre set up actually rode smoother and more comfortable than my 18" set up.
Thanks Zaq,
That's why I decided to save the S$4200 by not trading in my stock rims and spend it on other stuff instead.
Cheers.