I promised that I'll post a follow up on the ride quality issue with my E32 735i (no self-levelling) with BBS 18x8.5" RK (ET13) with 235/40ZR18 tires all around. For those who hadn't read the previous posts the reason I needed to wait till now to post this is that I have just replaced all aging shock inserts with Boge "automatics" (supposedly OEM quality?) and all strut top mounts.
When I got the car back after the shock replacement the ride was really firm...especially on rough concrete pavement the whole cabin vibrated like crazy. After about 3 weeks now the ride is much more supple. Although fast corners seem to be handled better when the shocks were "stiff", rough concrete pavement and all are now taken with much less cabin vibration and on the freeway the car is simply smooth as velvet.
So I think I am safe to say that the 18" wheels/tires does NOT really give a much harsher ride over the stock size, as long as the suspension is doing it's job properly.
p.s. For those who would like to see the 18" BBS RKs I might be able to borrow a digital camera in a few days...didn't get what I wanted from Santa! =(