Hi!
IMC I do let the cohort(s) suck XP, even if it's "against the rules". A character with a cohort is gaining more feats and hp out of the cohort - and sometimes even treasure. Furthermore, I calculate the cohort as regular character dividing XP and giving the cohort the XP as mentioned in the DMG. Some XP get lost - that's the price for having all the advantages of a cohort.