We will be indexing 400GB of data/day into daily indexes.
Elastic recommends Max Shard Size of 50GB to avoid recovery/reallocation issues. Putting that aside, how much slower would aggregation queries take? 20% longer, 100% longer? Cutting the number of primaries from 8 to 4 may offset some performance loss.
The aggregation queries have a constant-score filter with unanalyzed terms and date range clauses, aggs of terms over time intervals.