I agree, it should not be a problem with the current user base ; it was more an exercise in future-proofing (you may also call it premature optimization ... ) and the result of my recent readings about data modeling with cassandra. And I was more worried about spam detection systems (who usually do not like very large email batch) than performance.
 
I'll refactor that : weekly digests will be sent on monday morning for everybody but I'll keep the day  in the key so that we can easily distribute the load if it's need later on (I never really liked the reference to the current day in the Repository object anyway) .