there are no hidden perks as alluded to above.
see https://www.cathaypacific.com/download/eng-mpoguide.pdf
for the benefits of Diamond membership.
What is of more interest is, what happened to Invitation membership ? in the old days a couple of years back when you reached 2 million kms you were made a Lifetime member. Well they did away with that and replaced it with "Invitation Member" Having flown more than 2m kms and being Diamond for 4 years and MPO for 22 years, well I do not even get an invite to their Christmas party.