Seems there is a considerable £££ difference between 18" rubber and 19" rubber. Genuine M3 18' can be had for peanuts and 18" R888 for around £600 as oppose to £1200 ish for 19" R888. Cups about the same in 19" CSL sizes.
R888 aren't meant to be that good on a CSL, so little difference between the 18 / 19 cups