Standard sizes are 18 x 8" ET47 front and 18 x 8.5" ET50 rear. Assuming that the ones you are looking at are ET27 and ET44 the fronts will be stick out by 26.3 mm and the rears by 18.7mm. As for new tyres 235/40/18 front and 265/35/18 (presumption that they are 18" CSL replicas) these will make the overall diameter bigger so your speedo will be more accurate. Due to the extra width and offset you may encounter rubbing on the front, however if they are CSL replicas I'm sure you wont be the first to fit them, post a link to the item and someone with relative experience may know more
they are M3 fitment, so unless your fitting stretched rear tyres (ie 235/35/19) then they'll stick out like a sore thumb. with a correct size 265/30 theyll rub like buggery.
I have CSL reps on my coupe with a 265/30 tyre ET33 rear on a 9.5J Rim (Which is 6mm further in the arch) and its close.
Thanks for the advice Jamie and Sars, i see what your saying. maybe i don't think i will bother then as i don't want rubbing and i didn't really want to have stretched rear tyres.
Thanks for the advice Jamie and Sars, i see what your saying. maybe i don't think i will bother then as i don't want rubbing and i didn't really want to have stretched rear tyres.